I think the unboxed arms might flex a little, but I don't think you will get much flex out of a boxed lower control arm other than from the rubber bushings. You don't want the rear suspension to be too stiff as it can cause steering issues when you go into hard corners. You can make the suspension too stiff and going with a large diameter rear sway bar can do this as there isn't the weight on the rear of the car as there is in the front to "work" the sway bar.Thanks for the info Jim. I'm definitely interested in things I can do to stiffen the frame. And, this is a good time to make that decision. I'll be checking this out soon.
The seller of my car had purchased a new rear sway bar but never installed it. I was planning on using it. I'll check out the boxed lower arms. Are there any negatives? I've heard before that the factory designed these to flex.
I have decided that I want to buy a GForce transmission crossmember. They are very beefy. That will also help with frame stiffening. Not only is the factory crossmember flimsy, it mounts in rubber at the frame. I'm scratching my head as to why they did that on these cars.
The rubber mounts on the crossmember are more than likely to prevent drivetrain vibration & noise from traveling to the frame - it dampens it. Some who use the harder polyurethane bushings/body mounts note that the car's interior can pick up more noise when compared to the factory rubber bushings.
The polyurethane body bushing can also help to stiffen the chassis, as would solid mounts. The complaint is that they can "squeak" due to the material used. I went with the polygraphite body bushings which are said to eliminate that issue as the graphite in the bushings acts as a lubricant if the body/frame flexes. I got mine from PST, but can't remember if I got the hardware from them or sourced it as a kit from someone else.