Looks like the previous owner did a good job with the power upgrades and the drivetrain. Saved you a lot of money in the long run. I would now address the suspension and brakes. Here are my recommendations to upgrade the rest of the car to handle your current power and make it safe to add more power when you’re ready.
Since this is a street car for you, you are going to benefit greatly (especially at that power level) by upgrading the suspension. I would most likely ditch the pedders rear drag springs if you’re mostly street but you can keep them in the rears if you dont want to rebuy. I would go with a set of Koni Sport adjustable shocks out back and koni sport adjustable strut inserts in the front. They are rebound adjustable. Full soft for long highway drives and feels like how the stock shocks should have felt. Give them a couple twists and you’re ready to carve some corners or drag race but are still compliant enough to not rattle your teeth out on a bumpy street or going over train tracks(unlike super stiff coilovers). I keep my Koni’s about a 1/4 turn from full stiff up front and a half turn from full stiff in the rear. I just love the way it feels after starting at full soft and working my way up.
Definitely need to upgrade your front springs ( or front and back). I have and always recommend Lovells heavy duty springs. I’ve got the 20mm drop lovell springs up front and stock height lovell rear springs out back. It provides a slight rake and keeps the rear high enough to wear my tires perfectly even and can load up the trunk and have passengers without scraping over bumps.
I dont think any gto suspension is complete without a set of hotchkis adjustable sway bars. One of the best upgrades for the gto for handling on the street, getting rid of the obnoxious understeer, and adding a greater level of high speed stability. Again they are adjustable so you can dial in the way you want the car to feel. I’ve got my front sway at 2 of 3 (or the middle hole) and the rear sway at 3 of 4 for stiffness. That rear sway really helped to get the ass end to come around mid corner and added a bunch of high speed stability. The front really helps weight transfer and makes the car feel lighter than it is when going from turn to turn.
Change out all the original bushings! By now most of your factory bushings are toast. The most important for sure are Front radius rod bushings. Lovells/whiteline have caster adjustable radius rod bushings. This was one of the best upgrades in-terms of steering feel. Less dead spot on the steering wheel, better turn in and way less understeer. I believe i have mine in the +3/4 position for the caster. Perfect amount of sporty feel without rubbing the front tires on the inner fender well. Do the rear radius rod bushing as well while you’re in there (where the front radius rod mounts to the front control arm). Replace the factory front endlinks with any aftermarket version and it will come with new endlink bushings. The factory front endlinks are weak and known to bend. New rear endlink bushings as well but no need to replace the actual rear endlinks. New tie rods and ball joints are easy and will give you piece of mind. Rear subframe bushings are high on the priority list. They improve the feel of the car and make everything more solid. It also helps with the dreaded rearend clunk when loading and unloading the drive train. Again go with Lovells/whiteline. They arent that hard to install. Rear diff insert bushing is cheap and will help the rigidity of the floating independent rear end. Oh and the strut top mount bushings and bearings should be replaced when doing the front springs and struts. Then the normal bushings like front control arm bushings, rear shock bushings if not upgrading shocks (rear shocks always come with new top bushings) and rear control arm bushings.
The prious owner may have already done this but upgrade the motor mounts and tranny mount. With that much horse power you’ll want everything to stay in place. Go with Hinson poly motor mounts and hinson upgraded trans mount. This will also give you more Instantaneous power transfer to the wheels (bye bye sluggish feeling) and help to dampen the rear end clunk further.
Upgrade your rubber brake lines at each wheel to stainless steel lines. For $99 you cant beat it. I run the DBA 4000 slotted rotors all around and hawk hp+ pads. (Ps, Never go with cross drilled rotors on this car, its so heavy they will eventually spider crack around the holes... ask me how i know...) Insanely better stopping power than stock and they will never fade like the stockers. The HP+ pads a little noisy when coming to a dead stop. If you want something quieter go with the hawk HPS. I ran those and loved them over the performance ceramics. I’m a corner guy so the only reason i went from the HPS to HP+ is i eventually found the limit of the HPS with this heavy car. The HP+ has a higher heat rating. If you’re not beating on it turning corners ect just go with the hawk performance ceramic. Way less dust and still a great improvement over the stock pad. The last uograde to the brakes should be a high temp brake fluid. I only use Motol 600 for the brakes. It will never over heat or boil, ensuring you have a always have a stiff pedal push.
You havent mentioned anything about a clutch... with that much power you are going to want a performance clutch. The great thing you already have an MGW shifter which IMO is the best short throw for the gto. I have one so i may be biased lol. For the clutch, in your position aka. Street driving with lots of HP, i would Absolutely go with the Monster LT1s twin disc. Its the factory clutch from a C7 vette upgraded from monster. It will have the holding capacity you need but with the pedal feel of a stock clutch. I wish i would have gone that route. I went with a monster Level 2 clutch. Its a full face so it slips good on the street but increased my pedal effort by like 25-35%. It also has the downside of NOT slipping at the track when launching with Any RPM. Its basically an On/Off switch which makes it tough to get out of the hole on a sticky prepped surface. The twin disc LT-1s will slip as much as you want it to. Another good option for a twin disk is the Mcleod RST. A lot of people use them but i prefer the Monster. If you do the clutch put a new slave cylinder in it while you’re there to avoid having to pull the tranny later. Also put a Tick speed bleeder on the slave while the tranny is out. This makes bleeding the clutch a 2minute job instead of a 2 hour job. But it can only be installed with the tranny out.
Thats most of the stuff i would do if i were you to get the rest of the car upgraded to handle the power you’re already making. Then you’ll be ready to throw that super charger on and not kill your self lol.
(Ps. I wouldnt worry about it too much as you already have an updgraded driveshaft but i had an aluminum 1-piece ds... great for drag racing but on mine cruising at 75-80mph the ds would vibrate and by 120mph it got so bad it felt like the car was going to shake apart not to mention it wiped out my rear tailshaft bushing on the trans. I since have gone with a DSS 2 piece rated to 1000hp. One of the greatest upgrades i made for smoothness of operation. Handles the power but feels like stock and is good up to and past 160mph with no vibrations.)
2006 GTO M6
Motor - Stock with Vararam
Bunch of suspension
Bunch of brakes
12.81 @ 110.6 2.03 60'