If I buy a new tranny for my GT, I'm going to rebuild my existing one. I've seen people getting $400-$500 for rebuilt T5s on Egay and if a rebuild kit is only $170, then it makes sense to give it a try.
I took a tranny class back when I was studying auto tech about 10 years ago. It's been a long time, but the manual I rebuilt actually worked. The 727 I rebuild worked too, but the super-fsking-complicated 440T4 definitely did NOT. Thank God they didn't have a dyno to test a FWD transmission.