I've been thrashing Tokiko Illuminas with Ground Control springs on the track with no problems for almost two years. When they went on, I leveled the car front-to-back and lowered it about an inch or inch-and-a-half. I just lowered it about another inch, plus I've added negative camber on the back to give it some help turning-in.
I don't know the spring rates. The system that was put together went onto my everyday driver 1992 Honda Civic that was also an occasional track car, so I guess you would call it a "street lowering," not race-preparing the car. Now the car is pretty much retired from the street and I'm tripod-ing more and more, so my (new) coach/mechanic and I are discussing shock/spring/swaybar options that will decrease the body roll. He's partial to Koni yellows, says he's used them with great success on a number of Civics over the years...
I talked with a USTCC guy at Infineon the other day who did not especially like the gas-charged Ground Control shocks on his Civic, complained that they leaked a lot.