Does anyone know what the pack is rated for discharge, C?
When I purchased my WE BD36's about a year and a half ago, their brushed controllers went to 35 amps. Their standard battery was only 7 AH, so they were expecting it to suppply 5C. However, they were selling all their kits with the 12 AH pack "free", which would have been required to supply 3C. I expect the 7 AH packs weren't up to the task.
Anyway, I have verified the 35 amp draw during startup and hill climbing, using an inline automotive ammeter. All my batteries are still strong, but my normal daily rides are pretty short and the batteries are always promptly recharged.
Edit: Hey, its late and I already had to correct two numbers above. Hope it makes sense. My conclusion is you might get away with 5C, and will be okay with 3C. Of course the sag increases as your draw goes up.