What kind of catback do you have on your car now? or is it still the stock one... I run a prochamber on my car and they are awesome. I was always confused if I wanted an H for the sound or X for a few more hp but the prochamber is the best of both worlds. Any o/r midpipe is an improvement though. The prochamber is a little more expensive then an H, I think I got mine for $200 shipped which isn't bad at all. I'd definatley do the u/d pullies too, they are very easy to install. You'd be surprised with how much the U/D pullies will help you out. I think those are your best bets... that and maybee headers. You might want to think about headers now before getting your midpipe. If you want longtubes that is... if you want shorties then don't worry about it... good luck man!
For someone like myself that is not the most mechanically inclined, underdrive pulleys were probably the easiest thing to do, besides obviously an air filter. Just make sure to tighten your bolts to the pulleys after you put the belt back on. I forgot to do one of mine and the next day it got ripped off and broke my water pump. Its just like tightening your lugs again after you put the tires back on the ground. For the H-pipe, make sure to get the emmisions tube kit for it. Way back when, my friend and I put an h pipe on my 90 lx. We were running all day long to get parts because we were clueless. The kit is basically like a garden hose that connets your emmisions tube, which you have to cut off of your stock cats, to your h-pipe. Also, the night before, make sure to soak your bolts of the cat (from headers and from cat-back) with WD-40 or the like. It makes your life so much easier. Depending on age or rust, you might want to replace those bolts as well. I would suggest it. Flowmaster American Thunder is the way to go after that, just my opinion. Have fun.
I have a 95 gt auto and but far the best thing i have done to it was a precision idustries stallion converter. It dropped my times by 6 tenths of a second. I found mine used on ebay for under 300.00 bucks.
sage2k: about the manifold install, im not very mechanically inclined but i installed my new upper/lower (cobra) with my dad. It took us some time, but we got it in fine. All you need is tools etc. If you mark where every line goes, and get plastic baggies, for all screws and write where the screws go to you can't go wrong. Just get a tutorial on how to do it off the web. I used http://www.allfordmustangs.com/Detailed/614.shtml
The whole procedure cost me about $345 $290 for the manifold shipped, another 55 for the sealants, gaskets etc. It will speed you up, and it's a fun project to do, just have somebody do it with you, it will go faster Good luck!
I really think you should do FULL exhaust. I bolted on my catback and x-pipe and while I did feel a nice gain, I REALLY felt a power increase (as well as a beatiful exhaust note found only on 5.0s) when I installed my headers.
Also try shorty headers for ease of install, or even equals, do a search, you will find what you are looking for.