Hmmm, for launching a RWD car I'm not quite sure what kind of rates you would need. A stiffer front spring would transfer the weight faster to the rear, so that would help, but I don't know how a softer rear spring would react. In FWD you usually want a stiffer front spring to reduce wheel hop and to keep the traction down, so I'm not sure if you should stiffen both but, just have the rear springs not as stiff.

As far as handaling goes generally RWD cars still use stiffer front springs to keep body roll down so that seems right. Now as far as actual lbs per inch I'm not sure. You might want to see what the rates are stock, then see what some of the lowering spring companies offer and then go from there. If a company doesn't know their own spring rate then do NOT buy from them!

Ground Control is a good company and you can get custom spring rates in 25 lbs increments for no extra charge. I would check them out or even call them up and see what they say.