The low speed physics are not accurate in GT5, so more times than not you're better off just keeping it floored spinning wheels, it's faster, unlike real life. Trying to feed the throttle is pretty much useless, because if you don't have the throttle pressed hard enough the clutch won't engage properly, leaving you going nowhere and when you press the throttle hard enough for the clutch to engage the wheels spin. It's a pretty futile exercise trying to get a good launch in anything but 4wd. Probably one reason why drag racing was not included in GT5.