There is a lot of confusing information going on here, and most of it is not completely true. Mustang5L5 nailed it right on the head. To reiterate:
ALL V8 MUSTANGS FROM 85-95 GOT THE WORLD CLASS T5, EVERY DAMN ONE OF THEM.
The world class designation is nothing special. I don't care if it's a world class from an 85, or from a 93, they're all weak pieces of ****. They did get a little stronger in 90+, but you'll break it just as easy. the "z" spec t5 is nothing more than a standard world class t5 with the stronger "z" spec. It's still a damn t5, and it'll still break just the same as soon as you start powershifting it. If you want a real tranmission, stay far, far away from ANY t5, unless it's a gforce t5. world class trannies did NOT come with a steel input shaft bearing retainer, you order that aftermarket.
Additionally, to help the original poster, the t5 shifts as smooth as butter, and seriously, it's about as smooth as it gets. If your t5 is shifting notchy, it's likely because your clutch adjustment is too loose, and it's not fully disengaging. Tremec trannies are notchy as all hell, and if you're looking for a quick shifting alternative, stay far away from the tremecs and set your t5 up properly.