Nitrous is worth a ton of power for a little bit of money. The problem is that it runs out and it can't be used ALL the time. I'd consider nitrous as "part time" horsepower. No offense intended to those who have it, you can make some damn fast cars with it and it's one of the best "bang for the buck items" you can get.
If you want serious power all the time, you just can't beat a supercharger or a turbo. I'm not sure those count as slight engine mods though. They are "bolt ons" these days, with the many kits available. They're also not cheap. If you're handy with a wrench and a welder, you can make your own system for significantly less than the cost of a professional kit.
I wish I'd dynoed my car before/after I put the headers on, it FEELS like more than 30 horsepower. It may have finally allowed the tune and the CAI to work to their full potential. To me, that's the best mod I've done so far though.
Good Luck with your decision and please report back what you do and if it helps the car's performance.
