Is there any benfit to putting guppeys, bombers and starfers on a power zone?
Yes there is, read about the PZ here:
http://knucklecracker.com/wiki/doku.php?id=cw3:the_game (http://knucklecracker.com/wiki/doku.php?id=cw3:the_game)
So just to confirm, the forge is the only building when built on a PZ recieves no benefits?
Ore mines, siphons, and the forge gain no advantage.
Spoiler
The final titan costs half of what it normally would