The 280lb rear springs are way too soft. You need a lot of rear roll stiffness in any fwd car, and most real racing setups are very nearly equal rates f/r. I assume Ground Control sells the soft rear springs to produce an understeering kit for liability reasons, but I don't really know. I have been doing a bit of messing around with rates, but I still don't know what's best.
If you want to run on rough surfaces, you might think about something a good bit softer, like 300/250 or 250/200. On rough pavement my car gets very loose because of the stiff springs. You'll have to run the car higher to keep it off the bumpstops with softer springs, though: I have around 1.25" of travel before a 1" bumpstop now.
I went with the 16" wheels because they were cheap, and so are 16" tires. I've thought about 15" for lower weight, but if I don't want the taller sidewalls or lower gearing. 17" would let me run a 225/45R17, which gives huge tire selection, but that's going to be several lbs heavier than my 16s. I'm running a 205/55 right now, despite the tall sidewall, to see how the gearing feels. If I like the taller gearing, and the 1/2" higher CG over stock size doesn't slow me down, I may go to a 17" if I can find some light, cheap wheels. One other advantage of 17" is that 7.5" wide wheels are common, while they're rare in 16". Mine are 7" wide, wich is great for the 205s I've been running, but 7.5 would be better, especially when I finally get some 225s.
