They get that hot because they're used in a rather crude manner (directly dropping the voltage from a +12v reference) - a multi stage solution would reduce the heat impact on each transistor. One could browse electronics retailers to see if they carry higher rated MOSFETs but in any case with such a brutal, single-stage circuit, wou'd have to use proper heatsinks anyway. The ones on Kaze controllers arent bad by themselves, they just need proper cooling.