As some said, stock classes you will absolutely feel the difference between 40 and 60c. Mod you probably won't.
Unfortunately there is no industry standard for "c ratings" so one companies "90C" may be the same as another's "50C" It's internal resistance that matters the most and none of the big manufacturers list that ratings on their packs so there is no way to tell other than running one to see which works best for you.
A battery gets juice "pulled" from the motor/car, it does not "push" amps or volts into the speedo. A 6000mah 60c pack can safely handle 360amps. The only car capable of pulling that kinda juice is maybe a Toyota Prius. So your speedo and motor are never getting 360amps. But the motor may pull 80amps peak and your battery will be able to provide it. The bigger % your system pulls of available to usable = the bigger drop in voltage leading to shorter runtimes and less "punch" This all has to deal with motor, gearing, surface traction, etc. Offroad buggies pull far less amps than onroad vehicles since the traction is generally not there comparatively.
Hope this helps.