I also run Impact Panzer III/IV tracks on my HL P3 and Tamiya P4, great performance.
The Impact TUs are good when you are using the drop down gear for more realistic speed, they tend to be a bit noisy when new, and a little break in period is needed to get them to quiet down some.
I generally use the Tamiya gears from the stock Tamiya TUs to help the noise problem and even on some of my Impact TUs I have installed Ball Bearings on the gear shafts.
hope this helps.