I had a T5Z from MS that had 30k on it behind his 351 which preceded the 427.  I never ran it...sold it like a dummy...but he said it was solid and the guy that bought it never said otherwise.

BB1

The T5 has a largely undeserved reputation for being failure prone.  If you run a proper clutch that is adjusted properly, use a good shifter (I wouldn't run anything other than a Pro 5.0 on a T5), and don't have slicks on the car a T5 will last as long as any other transmission.  With driveline strength its all about the weakest link.  In a street car the weakest link is the tires.  You will break the tires loose long before you shock load anything enough to break it.  The higher powered the car the more this is true.  The only thing I will caution you on is that abuse will break anything.  If you think you're going to be powershifting 3rd a lot then the T5 will eventually break, because by third you'll have traction even on street tires.  If you shift normally and drive like an adult not a pissed off teenager it will be fine. 

Racing is a completely different set of criteria, though properly built the T5 does fine in racing as well.  I've built probably 30-40 T5s over the years.  I've sold them to guys running 383 stroker Chevy engines in 3rd gen Camaros, D1 drift racers, drag racers, etc.  With proper setup and aftermarket parts they hold up under these conditions fine too.  However, if you are racing in a class that doesn't mandate an OEM trans there are inherently stronger (and possibly cheaper) boxes you could run (Tremec TKX, 3650, etc.).

Long story short, I'd run it without a second thought in your application. 
 

Depends on your definition of street driven? If you include hard shifts up or down, wheel hop, and lots off drop anchor shifts going into corners you might have a problem. I have seen wheel hop kill T5's as well as lots of hard shifts. But, I have seed that kill other tranny's too!

I have a T5 WC (purchased new) behind my 351W that I believe makes about 400HP, with 3.89, 9"rear.  This is a street car, that I do get on sometimes.  I did install home-made CalTrac style traction bars as wheel hop was a problem.  As TKO posted, these cars are so light in the back that spinning the tires is easily accomplished, and that reduces strain on the entire drivetrain. 
Tremec offers the TKX5 now, that will handle a lot more power, and the cost reflects this option.

1971 mustang 351w with a T5 built by forum member works and shifts smooth .Using a pro 5 shifter Amsoil auto trans fluid. I don't hammer on it.And it works great by my standards. Fantastic rebuild.

One thing I will add is that thought the Z-spec T5 has a slightly higher torque rating than the standard WC T5 don't believe that means anything in the real world.  In actuality there is no strength difference between the two.  Torque ratings for transmissions are misleading due to the way they are determined.  Basically they lock the trans in 1st gear and shock load it until it breaks.  If that seems in no way real world its because it isn't.  You are not going to shock load a trans like that in 1st gear unless you have a super aggressive clutch, slicks, are on a prepped track where the slicks can bite, and have the suspension set up right.  I've NEVER seen a broken 1st gear in a T5. 

The only reason the Z-spec T5 has that higher rating is because the 1st gear is a lower ratio than the Fox V8 T5s.  That means the speed gear and the gear on the countergear are closer in size, so the speed gear is exerting less leverage on the countergear.  Therefore, in their "test" a lower gear ratio in 1st is going to perform better.  If I could build a trans where the 1st gear was 1:1 I bet a T5 could handle 500lbs/ft. in their "test".

Point being, don't get hung up on those ratings; they're largely meaningless.  The Z-spec has better ratios, but in reality isn't one iota stronger.  It also has the better 1/2 synchros if you still have the stock Fox era stuff in a Fox era T5, but if its been rebuilt all the rebuild kits come with the 3 piece synchros.