Couple of points for ya...
You will put more time and care into it than any shop ever would. I'd almost guarantee (if you are an anal Mustang Owner like alot of us are) you'll find some part of it that the shop didn't do the way you'd have done it yourself. It'll hang too low, or the tips won't be even, will be sticking out too far, etc. Been there, won't do that again.
If you need a new TOB, I'd put a new clutch it it.
If you need a new TOB or a new clutch, they'll likely be dropping the midpipe anyway. Having transmission work done is the perfect time to have exhaust mods installed.
This is a great DIY mod...it's just taxing on your back, but otherwise, the work effort itself is really simple and straight forward. Air tools help (nearly required on a midpipe) but are not necessary on a catback.
The LM2 is a GREAT catback - super high quality and I just loved the tone. My wife had it with an off road X on her 01 GT. The double wall tips are really nice. I've gone through a lot of different catbacks and the LM2 is the one I would buy again. Excellent choice my friend.
Do you have a midpipe other than stock? Once again, this is the time to buy and install one.
So there is my nickel's worth. Best of Luck.