I think Dempsey is suffering from the classic problem of signing for a new team under a cloud of being hot property and then having to live up to it. Once the pressure is off I would expect that he'll start getting more goals.
Personally, in our side, I want Ade as the starter and Dempsey as a supersub game changer. I think Defoe will get farmore goals from Adebayor assists than he will player with Dempsey, or, for that matter, Siggs.
By the same token I expect Dempsey will get more goals with Adebayor playing in the team. His main strength is making late arriving runs into the box and slotting away clinical finishes, so he'll function a lot better in the side with Ade holding up the ball and laying it off to him at the right moments. Playing behind Defoe, who as we've discussed a million times can't do that kind of stuff, he looks a lot more unsure of his role.