It is about time we post some 101's here on the forum
First of all the difference between the Mega motor with a ni-mh battery & a brushless BLX motor on a 2S (cell) lipo is night an day!
Now the bad part: which build (year) Granite do you have?
You might have heard the 2015 and newer models have no metal geared transmission anymore?
That means upgrading to brushless will mean also upgrading the whole tranny with gears and differential
If you have a 2013 build, it does have the metal gears.
An alternative could be upgrading to a modest brushless system: something on 2S and 2700-3000kV and 40-45Amp Esc.
This will give less strain on your composite gears and differential, but still be much, much more powerful and faster than the Mega brushed motor.