Hi WRX...
I also have the Integy gears and they're pretty light compared to the stock gears... I believe the counter gear (which connects to the spur gear) is of slightly different pitch(much finer) so you will notice a difference in "noise" compared to the standard TA03 gears. Speed wise (last time I ran my car was back in 1999!), I felt the car had more torque probably due to less drag from the thin diff gears.
By the way, this is a nice interesting thread... I'll try to take pics of my 2 cars when I get home... both are equipped with Cross, Integy, Pheonix and Tamiya hop ups... They've been sitting on my shelf for quite some time now and deserves much needed attention... Thanks for reviving this car...
-Ian