What speeds are you trying to obtain..use the speed calculator to calculate speed, try different pinions, etc...great tool to answer many questions. Can plug in 4s, 5s and recalculate. The 30c batts should suffice
http://www.scriptasylum.com/rc_speed/index.html
1. MBX-6 T8 1900KV, RX8 ON 4S
2. MBX-5T 1520, MMM ON 5S
3. MBX-5 ONROAD CONVERSION 1515, MMM ON 5S
4. MRX-3 ON ROAD CONVERSION 1512, MMM ON 6S
5. TEN T 2650 T8, MMP ON 3S