I'd keep Martin with Geno, for that purpose. Letang is a frustrating player to watch, he has so much talent, but he has so many issues breaking past his mental blocks for shooting and improving aspects of his game like his shot, passing, etc. He can rush up the ice like no one else in the league and deke through them like a skilled forward, he shoots well on the go, but the moment he has to think instead of react...game over.
Malkin's shot is as potent as ever, but people know about this and so they try to crowd him when he gets the puck and it looks like he is about to tee it up. He needs a good dish man for the one timer, Sid is good but most of the time, they're on the same side with Geno moving back and forth from the point, slot and the high right boards.
The thing is that the Pens don't even have the right guy for the point. Martin can make the pass precisely, at the right pace, and even disguise it. BUT, nobody fears him shooting from the point, meaning you can cheat towards Malkin.
That's what made Gonchar and Geno work so well. Gonchar would have his head up, looking at the net, looking for a shooting lane. You HAD to respect his shot. Then, the moment it looked like he was ready to shoot, he'd slide a perfect no look pass for a one timer to Geno who would wire the shot.
The other part that made the equation work was Sid. I know it sounds nuts, but Sid was an important component. With Geno on the right wall, Sid would play near the right goal line and set a decent screen.
Now, Geno's got nobody setting a screen and, in the rare event that he actually gets a perfectly paced/placed pass for a one timer, it's a telegraphed pass, so the goalie already is sliding and has nobody blocking his view.
Personally, to the degree that this explains part of the issue with his not shooting so much on the PP (and I think it's a legit explanation), I don't think it gets solved until/unless you've got Despres OR Pouliot at that point. Nobody else can make the pass and present the threat as a shooter.
A guy who knows how to work the front of the net and slide will the play obviously would be an added bonus, but IMO the best two forwards on the Pens at moving with the flow of the play to set a perfectly timed screen are Sid and Bennett. I suspect Kobasew would be in that ballpark too.