My reputation as a T5 guru is GREATLY exaggerated. I have had the same T5z for something like ten years now. I have just now opened it up and taken it apart to install a pro shifted G-Force gear set. The reason why my T5z held up is because of the clutch in my opinion. A soft hitting fully adjustable sintered iron disc clutch will shock the drive line a lot less and can be faster as well.
The "Rebuilding Your T5" video from Bob Hanlon is fantastic and makes things easy.
G-Force makes a stronger case if you're worried about your stock one.