Long story going to try to share some of my experience.
Back ground: I've had 9 mustangs. 5 Foxes from factory to two 8sec cars(1 turbo and 1 No2, 01 cobra, 07 Gt mostly stock, 07 Gt500 605hp 620tq. 01 Camaro z/28, 01 WS6 trans am.
I've made 420hp on stock drive train in a fox. I built the motor, using a stock block, HCI, Stroker kit, left the trans alone and put gears in the rear end. That was it and it was my favorite car.
Built a 91 fox lx with 363ci Dart motor with 88mm turbo, cage, suspension, 9" rear end, swapped a 3 speed auto trans. Ran 8.80s and sold it because of the trans I couldn't drive it on the street.
Built a 90 fox lx coupe 408ci nitrous car, striped, mini tub, auto, and ran 8.90s but couldn't drive on the street because it didn't turn worth a damn.
Built or bought a few others in between. For the street.
I nuked one AOD trans in a fox while running about 375hp high mile car and I knew it would give up at some point. I had it rebuilt and it worked great until I sold it.
01 Cobra was mild but the IRS in that car had weak bushings and the car had a lot of wheel hop. Bought it with 12k miles in 06 sold it in 09 with 60k
Gt500 was bought with 44k miles pulley kit. Started having grinding issues shifting into 2nd and had a ton of wheel hop sold it before it had 48k miles.
My two GM cars the trans am (auto) and Camaro (6speed): I nuked the trans and rear end in the trans am after a HCI package. I didn't believe everyone when they told me it wouldn't hold up because I made good power in some foxes before this. I didn't put the trans am on the dyno but we guessed the motor made over 400hp. the week after I got all this done I broke the rear at the track. After fixing that I was driving my buddy around and lost the trans. I went out and bought the z/28 after this did the same HCI package on it. Thinking that the 6speed would be better I lost the rear 2months after getting it running and gave up.
What does it all mean? A fox in good shape will take a good amount of mods and abuse. It's cheap, fun, 400-450hp limit on the stock block, but a little dated. 6speed F-bodies are good cars if you upgrade the supporting drive train before the motor. It is difficult to beat a vette in the corners.
PS. I'm about to buy my 10th mustang and It's a fox. It's going to be a long build from ground up and hopefully will win the 10.0 class in true street.