What gets me is the 2.3 was rated at 90hp and 123lb-ft with aT5 rated for 240 lb-ft, while the 5.0 was rated at 225hp and 300lb-ft with a T5 rated for 300lb-ft.
It's the actual gear ratio's themselves that dictate the strength, at least up to 1990 when I believe there was a material change. Up to 1990, the V8 T5 was actually rated at 265/275 tq. They didn't get 300 until 1990 with a slight gearing and material change. But if you took, say, a 1988 4cyl and v8 T-5, the internals of the trans are exactly the same except the gear ratios are different. Since the 4cy gearing is shorter, you get a bit more mechanical advantage with the gearing which is why the rating is lower.
I believe the 93-95 Cobra T5 gained 10 ft-lbs of torque rating simple by the addition of the pocket bearing design that helped reduce mainshaft spread. Of course, this is speculation, so don't quote me on that when this comes up in an internet search. Typically 3rd gear tends to be problematic because it's the furthest gear from any sort of bearing support. Under load, gears tend to want to spread and 3rd has the ability to move away from each other slightly. As gears spread, the teeth, where they contact, become thinner, and the risk of shear goes up. One way to improve T5 life it so stiffen up the mainshaft supports a bit. This might be a stronger case, a stronger mainshaft (which astro sells) or even the cobra pocket bearing mod.
Of course, 1st gear also has it's own problems with shearing the teeth off the input shaft/1st with a hard launch. But, this is gearing specific. If you had some crazy gear ratio of 16:1 for 1st, you could probably shear the teeth off with a stock 4cyl with a good clutch drop on sticky tires. The Z-spec's 2.95 1st gear is probably a good reason to the increased torque rating of that specific trans.
Disclaimer, i'm not a transmission expert, just someone who's rebuilt a few dozen or so on his own workbench. Take this all with a grain of salt