Mmmm, would guess 20 or 30W for that heatsink, offhand. Varies widely with how much temperature you want to allow it to reach, and orientation and airflow.
You *might* be able to reach 200W if you use thermally conductive epoxy to attach water cooling tubes to the bottom side. (You still need to cool the water.)
In power levels of this range, it gets very attractive to learn how to design and build switching supplies. It's a little bit more complicated, and opens up your knowledge base to a whole lot of modern electronics.
Tim