My friend runs a 10.5T tekin system in his 2wd b4.1. On the outdoor track its only marginally slower down the straight than my 6.5T tekin system in my 4wd b44.1.
He may be overgeared a little, but there isnt masses of difference.
If you get a good quality system you have the option of limiting current, increasing timing, boosting, allsorts infact.
So my advice would be to get something middle of the road, between 8.5 and 10.5 then adjust the speed to your liking on the esc. An 8.5 will be as fast as you will ever need in a 2wd in my opinion. Likley too fast at first, but you can turn the power down
