Thermal Pad Tests

Excellent report and good work. Time to go and order some Al2O3 insulators.

Regarding the mounting, there is several appnotes from various manufacturers showing that clamping reduces thermal resistance between body and heatsink of the TO-varietys due to less physical deformation as compared to when using mounting screws. Especially for the mounting tab types.
 
That depends on the TO variety - its not the case for TO3 for instance! Single mounting screw versions its certainly an issue - especially if you over-torque the mounting screw and bend the tab - since the tab is soft copper this is an easy mistake to make.

The MT200 package design with two mounting screws on either side as partly I think to address this issue.
 
Mark, you are absolutely right, thanks for the clarification.

I actually successfully mounted TO-264s on a predrilled heatsink (TO-3 /TO-204) for reuse in a variable power supply. I used a bar and screw+nuts to press them against the sink. Work just fine.

(I actually used MJL21194s that was reused from a blown power supply, ie blown as transformer meltdown)