longtubes do help, but i'd upgrade the rest of your exhaust and intake parts, then get a new clutch before LT's. (most LT's require you to remove them in order to change the clutch on our cars, and LT install is a pita on 2V's) but you will get a power increase across the board. shorties dont help near as much as LT's because the factory manifolds arent too bad. they arent great, but there have been worse.
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.
If you are going to go through the trouble, make sure you know what you are getting yourself into with LTs. I have had to remove one of mine to change the clutch already, it turns a 3 hour job into a damn near 13 hour job. They are great as far as power goes, you can tell a huge difference in the torque curve and pull up top. I prepare for an entire weekend for the install if not more.
I have a full Bassani exhaust w/cats that use mid-length headers.Hooker,Kooks and Bassani are the only ones that will allow transmission work with the headers in place(I believe???)Bassani headers also do not decrease ground clearance and are 50 state legal.According to MM&FF you get 17.3 HP and 17.4 ft/lbs of torque on a stock GT with a full Bassani exhaust w/cats.Shorties are no easier to install than long tubes and if you know what you're doing will still take several hours.I gave up and had the car towed to the dealer and paid $620 for them to do the job.Labor time was six hours for two techs (there were actually four) and they had all the right tools.Lift,engine support,and previous experience.
Lts is the way to go if you're going to do it. Install can be a PITA very easy. If you do it DROP THE K-MEMBER.. unless you have access to a lift.
I lost some low end power after installing the full exhaust but back when it was cold when they were first installed the car would lose traction at 4k and try to get sideways.. it was pretty cool. After having the car dyno tuned I gained back a lot of the power I lost down low.