does anyone use really big batteries?

[Finnster]

out of curiosity, how much were you quoted for the big MA pack?
As already said, the "C" rating can be real BS at times depending on the company as there are no real stds. They are not necc lying, but they may be "optimistic."
The only way to really tell is to see discharge graphs and compare performance, as well as anecdotal reports. If a battery is getting hot during use, its pushed too hard.

If you want 60 mph out of a Muggy, then you are going to need a lot of power. I would plan on 2500-3000W peak or so. For 6S, this means about 120-150A peak. If a batt truely does 125A continous you will be fine. However, 3000W out a 1515 is going to start pushing the motor perhaps, esp on a hot wind like a 1Y. If you don't already have the motor, I may think about a 1521 even so you don't possibly overheat and/or use thegreater TQ to gear up and hit your spd goal. 60mph in a big MT is really really fast.