Did you check your setup on the top speed calculator?
http://www.scriptasylum.com/rc_speed/_top_speed.html
If the Genesis uses the 1.5mod 26/9 f/r diff gearing (I'm not positive it does, though), then your top speed on 5s might be around 25mph with that motor and gearing.
I wouldn't skimp on batteries, especially with Zippy and Turnigy 30c and 40c so readily available now.