Give a nice flat mosfet package and a flat heat sink, I wonder how much benefit accrues from adding silicone grease. It's really messy in production and it's hard to confirm proper application. A little googling didn't provide hard numbers.
I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN or mica insulator, no grease, 40 watts. I guess I'll have to try it.