I would definately have to say yes for the overgrip. I do have a good basis for this statement, because when I got my POG I played with the straight leather for probably 3-4 months. However, I rescently switched to a tourna grip overgrip, and I must say, It's an improvement:
My hands stay dry, even when the rest of me is soaking in sweat (which does happen to me a lot)
I have a better hold on my raquet, resulting in better stability (especially noticable at the net, where the POG's lack of stability on off-center hits can be a problem)
I also like that with the overgrip I still maintain the excellent "feel" that the POG has, with the overgrip not causing any real dampening at all.
For me, at least, the choice is clear.... with all these pro's I've yet to find a con in using an overgrip on leather