I run a RX8 2000 combo in my 6T on 4S. It's real reliable.
I used Brian G's calculator to determine what Tekin system I was going to buy. I configured it out to what motor I needed by finding the right rpm range for best efficiency. The 2000KV on 4S is 30K rpms. Perfect if you ask me. Low temps and great run-times geared 46:16.
You might want to check his sight out:
http://scriptasylum.com/rc_speed/_top_speed.html
Hope this makes sense.