This thread hurts my brain.
How do you expect anyone to provide you any help with the information you've provided us? A RWD muscle car with 315rwhp is really nothing to brag about btw, but back to the question.
Should you keep the 08 WRX or sell it for an RS or an 07 STI, hmmmmm. Let me ask you a few questions.
1. What's the purpose of the car? (daily/track/strip/autox)
2. What' your budget?
3. Do you own the WRX or are you making payments?
There are many modification paths that you can take that will make the car significantly faster in a straight line (upgrade turbo, turbo back exhaust, injectors, fuel pump, tune for e85), and will cost far less then the price difference of your car and the sti (easily 7 grand or more). If you think the WRX is slow why are you even considering an RS? I'm just having some troubles trying to figure out exactly wtf you're trying to do with the car.