Fastest BL motor on 6 or 7 cells?

[SpEEdyBL]

Don't get the xl5000. You won't need it. Many people underestimate the power of the basic 5300. You should be able to do rolling wheelies at 30 mph. It's been done in a stampede which has similar sized tires.

If you stick with your 8s, gear it way up. It reaches it's max efficiency at 79 amps at 7.2 volts. (When iron loss = copper loss)