I don't know if that ESC will do the job. I have heard the same general dislike towards them but have no experience, and can't really comment. Statistically they should handle two 775s fine. Statistically. Two TZ85a speedos from Botbitz would do it.

As for speed, I don't know how big the driving wheels on your tracks are, but at say 80mm, you'd get about 11mph/18kmh. That isn't crazy fast. Many machines are that fast (Tormenta 2 does about that) and faster. I imagine it'd be more a worry of if the added stresses from the speed increase would cause problems with the tracks. Skidding sideways to a halt from 11mph will make the tracks work harder than from say, 5mph.

The Magnum gearboxes and motors would garner around 8-9mph, again assuming 80mm driving wheels and 18v. They're more expensive but potentially the better option, they look a fair bit chunkier and the added power/reduction won't go amiss when shoving a tracked machine around.

Plus, more speed = more cool factor, right? Right!