Does the dish shaped head significantly increase effective piston area? Not a bad idea...
I asked a few friends about this, they all agreed that the force only acts on what is perpendicular to it, meaning no difference. However I did find an old "google answer" which I found interesting:
"The pressure acts on what you might call the projected
area of the piston. The projected area being the area that would
project onto a plane perpendicular to the axis of the piston."
I'll be sure to post any more info I find.