Don't really think the "Mach 1" transmission is anything special; it's the same TR3650 used in the 02-04 GTs. 47-lb injectors on a normally aspirated 2V??
The price doesn't seem unreasonable but there's no reason to not make a lower offer on it.
TBH, I wouldn't. I have a 2010 STi and actually prefer it to the Mustang (sorry...)
You say the Rex is immaculate: If it was a beater POS I'd say go for it but unless you hate your blobeye for some reason, I don't see the car you're talking about being an even swap for the WRX.
8k for a GT? It needs to be blown or very nicely built. And as stated, a mach 1 trans is the same as a GT trans.
Those injectors alone would scare me off. It has to have had forced induction or n2o on it at some point. If not, then there is absolutely no reason for those injectors. Most 5.4 swaps don't make much if any more power than the 4.6 (esp stock for stock). They have the same heads (again stock for stock) and the longer stroke etc means they live better in low rpms and make torque but suffer up top. Even a 4v swap the 4.6 makes as much power and revs out better.
I would not make the trade if your subi is very clean. I don't see you coming out on top in this one.
EDIT: Looks like I'm too late to the party since I see you posting about a trans leak on a mach 1 trans on a 5.4 car.