Then perhaps you need to adjust the rear more stiff. The front is light because the car is squatting under acceleration--perhaps too much. I'm assuming at 120 mph (constant) the front isn't light (as the weight returns forward)?

Have you played with front ride height, to adjust perhaps 5mm lower than the rear to increase rake, which would decease the amount of weight transferred to the rear under acceleration?

My 997.1 (front end) never felt light under acceleration, although it (front end) did bounce over bumps.