I run a gear diff in my T4.

I tried it first at a large dusty outdoor track with an 8.5 motor. I attempted tuning the diff with 30 wt shock oil, 5000 wt diff oil, 10,000 wt diff oil and ended up at 30,000 wt diff oil (as recommended by Xtreme RC Car test with the HIP Blitz) but it was always loose, especially on exit of low speed turns. The gear diff was a disaster with a mod motor and low traction. My T4 performed a lot like my old HPI Rush did!

I'm not one to give up so I also tried the gear diff at a high traction clay indoor track with a 13.5 and I was surprised to find that it actually performed very well. My truck turned in much better with the gear diff and traction on corner exit was not a problem. So, I'm going to leave it in for as long as I can this winter mostly for convenience. But somewhere in the back of my mind I know the ball diff would be slightly quicker.

Based upon my experience, I would advise to leave the gear diff in if you're running stock short course (you have to anyhow) or even 17.5 or 13.5 short course, but, I would not want to run a gear diff with a mod motor in a 2wd vehicle.