20 should be OK, 19 fits on mine with the whiff of a dremel past the bulge for the 4th internal gear, there is loads of material there so it's fine to do IMHO.
Ref the heatsink, it's only Ellis that is such an animal he needs to run it (actually Nathan Ralls for similair animal tendancies
) For most of us it's not really needed. That said Ellis doesn't run any front window removed, if anyone ever has any heat issues, just open the front window up a bit to pull air through and it makes a significant difference!