I know this is a fairly old thread to be contributing to but I want to add that I am a fan of Sbarro pizza and altho the original poster was mentioning Shakey's, I might add you might want to try out some scamorza cheese. I did some searches here and Peter gave mention to some places using blends of cheeses and I forgot which chain he was referring to but he mentioned that a chain (maybe it was Papa John's or Sbarro or Domino's but I don't think Shakey's) uses a blend of part-skim milk, whole milk, and another cheese. He was not sure but he thought maybe a cheese called scamorza was the right third cheese.
I used to go to Shakey's a lot as a kid since it's only about a mile from me and I still am here as an adult. I don't go there much now so I can't really help with Shakey's but Sbarro cheese seems to be really tasty but the appearance of the cheese seems to look dry and a bit plasticy. Now, not to scare anyone from Sbarro, I love the taste and the texture of their cheese regardless of the appearance of the cheese. It's not a gooey mess like many pizzas I eat even pizzas I love often are gooey messes but this Sbarro is cool the way it appears. If I recall Shakey's kind of looks along those lines too. And because of Peter's suggestion that maybe scamorza is a cheese that some places use, I did research and found that the description seems to fit the bill for the look and taste of Sbarro, therefore I am going to make a blend of part-skim milk mozz, whole milk mozz, and scamorza. I just have to figure out the proportions and taste it to tell if it nails the Sbarro taste. From my memories of Shakey's the cheese taste different than Sbarro BUT with that said the texture looks similar so I wonder if they also incorporate scamorza with mozz but different proportions than Sbarro.
Bottomline is if you can find scamorza in your area go for it and give it a go and experiment with blending it with different cheeses and you might find what you are looking for. I am going to do that. The Italian place I go to is closed Wednesdays so on Thursday I shall go and buy some.