Well I'll take a shot at answering this...
IF your currently using a 108t spur and your final is around 8.0, then your likely using a 30t pinion. (108/30=3.6*2.25=8.1 final)
So if you drop down to a 100t spur and keep the same pinion, you'll end up with a final of 7.5(100/30=3.333...*2.25=7.5 final)
The result of this is that you will have more heat in your motor and speed control, and the ratio is probably a bit too low for a 12turn mod.
Now if your trying to go to a 100t spur while still keeping the final around 8.0, then you'll have to go down on your pinion size to something like a 28t pinion. That will give you a final right around 8.04.(100/28=3.57*2.25=8.04)
The only concerns that you really have with picking a spur gear is:
1. does it fit (most spur gears fit the Tamiya mount)
2. what ratio do you want
3. can the spur and pinion fit with the size gears you've choosen. In other words, will a small spur reach a small pinion and/or will a large spur still fit with the large pinion you want to use.
I hope the answer you were looking for was somewhere in all of that.