T5's are very strong for a compact manual transmission. They only get a bad name because of guys picking up junkyard units and expecting them to last doing clutch drops and speed shifting on the drag strip with a ton of grip, and that gets even worse when people are buying them second hand and they have no data tag on them to indicate they came out of a 4cyl Mustang.. The weakest of the T5's used in Fords.
They're more than acceptable behind a mild V8. I've read numerous stories on several Mustang forums of guys picking up used T5's and flogging them on the drag strip and getting around 2 seasons out of each unit behind a 300-400+ HP V8.
As I said, the most important (and cheap) upgrades a guy can do to increase the longevity of the T5 is to install a steel front bearing support and a steel rear cluster gear support plate. T5's exploding comes from the rear cluster gear bearing race being constantly driven downward/outward. The forces of the gears trying to shove each other apart under power eventually eggs out the aluminum case around the rear bearing race. The gears start to drive apart to the point it knocks gear teeth off, jams up the gears, and possibly explodes.
This guy probably knows more about T5's than 99% of the people out there:
He has several videos on installing the G-Force gear sets for higher horsepower applications. He also has videos on Sierra Cosworth T5 stuff..
GB