3" tubing is overkill for 120hp. But if you can fit it, it won't hurt beyond a little lag.

2.5" is about optimum between flow capacity, space occupation and turbo lag. Especially with the stock intake, it will restrict it back down to 2.5" anyways.

I'm using 2" tubing for my air/air system. However, it will cause the turbo to surge at low RPM's from it's lower flow capacity.