A 5.5 may be a little too hot, even on an open track. Anything between a 6.5 and 8.5 should be good in a 2wd buggy. If you go for a 7.5 I might be able to help you out:
http://www.oople.com/forums/showthread.php?t=56178
If it's too fast, the software allows you to turn down the throttle limit.