It really depends on how good Toomey is. If he really sucks as a campaigner then Sestak will roll him but if Toomey proves to be a good candidate then it will be a fight. Let us remember that when Specter ditched the GOP Toomey strategically began to move to the center especially with his comments on Abortion and other issues. The guy isn't some knee jerk ideologue otherwise he wouldn't have done that.
It also depends on what issues dominate the campaign. Obamacare is opposed by I think 10% or slightly lower margin in PA, and Cap-n-Trade won't exactly fly out in Western/Northeastern PA. Some of Sestak's votes would certainly help Toomey in areas of PA.
As for the Philly burbs, if Abortion dominates the debate then yes, Toomey would get crushed, but in this environmment I don't see that happening, Economy, taxes, Deficits, Healthcare, Cap-n-Trade, and Immigration will all be bigger issues. And then it goes back to how good Toomey is at forming a winning message and sticking to it. Lets not forget, that Toomey has been leading for the better part of a year now in PA against both Specter and Sestak, that he was leading at all in PA, is somewhat of a miracle in and of itself.
The rule is, republicans generally need at least 25% in Philly and they have to at a minimum, split the burbs surrounding Philly county. In Pitt, Toomey will do slightly better, but he will get mauled in SE Pennsylvania.
If Sestak has any type of moderate appeal, he will also eat into central PA too.
Sestak is by far harder to beat then Specter, but lets not underestimate Toomey here either. He finally has his shot and I don't see him letting Sestak take this away from him without a big fight.