I'm using the suppressors to deal with flyback when switching inductive loads, as per my other thread you've helped in - hence the power dissipation.
The MOSFETs have an on resistance of only a few mOhms, and are only switching ~5A at the 10s of kHz.
The IGBTs are driving inductive loads, at currents of ~10A up to around 75Hz. The h-bridge is a VNH2SP30, so no - no bipolars here thankfully! Stepper chip uses MOSFETs in the bridges too.
The problem will arise in the future I foresee, with up to 20 high side switches potentially, so this thread was more to get a feel to see if the solution was viable re cooling.
Cheers!