First you guys are nuts. Whats the point of getting your racing buggy to hit 60mph? Oh well speed is fun ill admit.

I could be wrong but the main difference between 4s to 6s is more cells providing longer run times.

The mah should still be the same 5400's ex. This is the amount of run time you will get.

Your batt will also say 35c-50c this is the punch or when you first pull the throttle.

As far as speed yes batts have something to do with it but mostly from your ESC and or Motor's and Gearing.

With my 2250kv motor I run 15/46

Keep in mind my motor is way to big for racing most are running 1700kv's

To reach top speed. " im guessing here" try something like 30/46 or even change your spur gear remember smaller spur gear larger pinion.