You don't always want more grip. If you were to lower the roll centers too much, you could get the car really pushy. You'll hear people say the car is too stuck. Basically the car is not free enough to flow through the corners. Having the roll centers raised a little will usually help free the car up to flow through the corners better. If the car is pushing on entry, lower the front roll center, if you're loose on exit, lower the coll center. The opposite is also true, too aggressive on entry, raise the front, too tight middle to exit, raise it a little.