650 is the max spring rates that those type of shocks can handle with out being under damped, any higher and you should/would need full race shocks.
(eg my car 800/900).
your black car (very nice) is your daily driver.....could you switch suspension from one track day to the next track day...or even at the track?(too much pain in the a**s).
I would try gc's first because it's easy and has the higher srping rates(eibachs ),what tracks do you go to?