Not necessarly. 7s1p shall work too: 1400-1750 Watt peak ... more then enough. Runtime will be reduced though and depending on your gearing you will work them hard with your big motor. Even if you deplete those A123 in 5 minutes, they will take the "abuse" and still provide a few hundred cycles.
Here are a 2 videos of an E-Revo with centerdiff (needs thicker diff oil) with 8s1p A123 (they fit in the battery trays 2x 4s1p), Neu 1515/2Y (1100kv), MGM 14035. Spur 40 / pinion 18. Runtime between 7-9 Minutes. Recharge time 15 minutes.
Run on Grass:
http://www.braintrust.at/2007/ds/rc/revo_wiese.wmv
Run on a track:
http://www.braintrust.at/2007/ds/rc/revo_track.wmv