One of these days Tremec will make a close ratio gearset. In the meantime 1st is too low, 5th is too high, and the differential between the T5Z, and any of the Tremecs is marginal.
After extensive discussion with Joe Liberty of Liberty Transmissions, went with blueprinted T5Z (2.95 low, and .80 5th).
The gear ratios are all right with a 3.55 rear. Trans shifts like a Honda using
synthetic ATF. The way I drive the car (daily driver, tweaked 351), trans should last forever.
Don't monkey see, monkey do, because everybody else is doing it.
T5Z is a great trans for a street Cobra. All you've heard about T5's breaking was with 1,000 lb heavier cars, and OEM T5's. The T5Z is not OEM supplied as a production Mustang trans, so you won't find them in junkyards.
Seriously consider the T5Z as opposed to the Tremec.
Me? It's great!