Maybe not. 1 oz copper is about 70 k/w per square. I can probably surround the dpak with 12 or maybe 15 squares of copper before I hit the sea of vias, so the spreading part could be maybe 5 k/w. With a lot of solder-filled vias, I could possibly get the net dpak below 10 k/w. The trick would be to get the vias very close to the part tab without annoying manufacturing too much.
Then if I dissipate 8 watts, it will only rise 80K!
We'll try it.