[Alan Roehrich]

Joe, are you comparing a piston with a full round dish to a flat top with valve reliefs? Or are you comparing a piston with a full round dish to a piston with a reverse dome?

Provided the actual compression ratio is the same, there's nothing more efficient than a flat top and a decent chamber design (meaning the head has a chamber and both a squish side and a quench side).

A full dish is much less efficient, even with a good tight chamber, because the full dish prevents the squish and quench parts of the chamber from working.

A reverse dome is only somewhat less efficient, because the reverse dome design takes almost full advantage of the squish and quench parts of the chamber.

IF the head is the EXACT same head, and the cam is the same, and the ONLY difference in the compression is the design of the top of the piston, then the dish piston engine will make less power. Provided all else is equal as well.