I have seen optics mounted that way in NRA precision pistol (rimfire). I have not seen them mounted that way by anyone in that sport that I'd be interested in emulating. Something to note is that it's quite common for pistols to have full-length rails (making such a mounting easy), and the competitive guns are all blowbacks. I've tried mounting some tube-style 'dots over the ejection port and found the position to be...awkward, I guess, even when the weight balance wasn't noticeably different. I've also mounted Ultradots with both rings on the forward portion of the sight, allowing the rear to overhang the back of the pistol--same problem.
Upon trying out a red dot on an N-frame revolver, which placed the 'dot forward of the rear sight, I encountered the same problem, exacerbated by the fact that a revolver is further out in front of your hand.
I would say that the ideal placement is right above the grip. Moving the sight forward or rearward causes the entire sight move out of the line of sight under recoil.
Because P-F isn't shit.