LT's definate SOTP gains. No dyno to back it up, but others have dyno's that show decent gains of 11hp and up. It's gonnna get expensive. If your thinking of doing the rest of the exhust first remember that you'll need a shorty x/h mid pipe. I picked up a shorty prochamber off the trader section here for cheap. The cat back can be done first, but if your going LT's, wait to change mid pipe instead of buying it twice. If your gonna do it yourself that will save some money, but if not, call around and get some quotes as to how much for install and ask if they have done LT's on a stang before. The place i went charged me $300, but that was because they thought it would be like putting shorties in. They never did LT's before. I had to take it back 2x's to fix a leak. Get it done right the first time. Same thing with headers, don't cheap out.
Good luck.