But since a Miata weighs less the a mustang, the T5 should but up with a lot more power, right?

Yes.
The domestic worlds views on transmission strength is very, very different than ours.
They're breaking T5s with 3000+ lb cars usually running on massive sticky tires and being as mean as possible to them.

Back in my mustang and Camaro days, t5's were a frequent fail part but so was a 700R4 auto. V8 torque and the weight of the car have to play a significant factor in durability.

The T5 should stand up much better considering the weight of a miata and the lower low end torque numbers of the turbo cars.

The "wide range" Quaife (QKE3K, 2.564 1st and 0.794 5th) is still available and will be available for at least a couple of more years (according to the Quaife rep I talked to at Autosport last weekend). £1476.00 plus VAT isn't dirt cheap though.
With the Quaife and a 3.6 you get 1st identical to a 6speed 2nd with 4.1.

With my 4.875 I would with a Quaife set get a 1st that's taller then the 6sp with 3.6 (which I know is on the short side even with stock power from the 03 I had to 8 years). But I'm far from the torque range to have issues.

A 1st of 2.2-2.6 seems to be what's wanted.

Edit: What about this wide gear set for the T5?
http://www.gearboxman.co.uk/infot5.html

Ok, getting a new box and then start to modify it adds up. I'ts easier to convert a 6sp to a 5sp with a dogleg "1st" :)