Hi Chris
I don't know the internal ratios of the rc8 but wouldn't have thought it too different from the hot bodies D8/vorza, but for racing i used around 18 or 19 pinion with 46 spur when on 4S, also using a 2200 motor.
I would have found 6600 battery to big/heavy, handled better with 4500
there's a link to a little gearing application i wrote on this web page which would help if you want to gear for top speed
http://www.prcc.co.uk/forum/viewtopic.php?f=4&t=2654
HTH
