ianp:
If you want to use 20t pulley you have to also change those diff pulleys to 34t so the belt is not too loose and the overall gear ratio wont get too low (1,6 with 20/32).
The advantage you get in FK05 with its pulley size(20/32) is that the drive train is even smoother than in FK04 or T1R. But I think you can get T1R drive train very smooth and free with 1,77 gear ratio too. In my opinion T1R and FK04 have VERy smooth drive train and is very competitive to any other car in market.
If you want to get it running more freely, you can take inner covers off from the hubs bearings. It helps a bit. But then bearing needs to be checked more often.
One good tip or thing to take care of with T1R is that when you put the upper deck on, the chassis is flexing a bit so you can adjust the belt tension to very tight or to the right tension by right assembly method. If you dont 'flex' the chassis when you put the upper deck on, the belt usually gets very tight and the drive train is not very smooth and free. A bit hard to explain but I hope you understood..
Hope this helps.
BR,
Vesa