Basically, there's no doubt about that the ligther the drivetrain is (lower rotating mass), the more acceleration you'll get.
However, both the diameter of a pinon and the weight of a pinion is pretty small, so the advantages will be very small, compared to what can be gained elsewhere.
When it's about building up inertia in the drivetrain, I dont know in which cases it might be an advantage. I dont think it'll be an advantage to a touring car, at least I've never heard of any examples. There's a reason for making wheels of lightweight plastic instead of hardend steel
Gear mesh might be an issue, I believe that larger pinons and spurs have a smoother mesh. But again, I think the differences is pretty small compared to what can be gained elsewhere.
Weight distribution makes a huge difference. The motor represents 10% of the cars total running weight. So a movement front/rear of 5 mm, will make quite an impact on the handling while entering and leaving corners. Not to mention, that the armature is the single component, that by far represents the heaviest rotating part in the car. This also influence the handling, when it's about front/rear weight transfer. Thus the placement of the motor gets even more important.
If you're looking for pure acceleration, you might be interested in turning your TC into a regular dragster. However, depending on how drastic you're doing it, it will probably not be legal at a TC track.
If you're racing on a track, it's more important with a good setup, that suits both your car, the track and you as a driver.