Although you may have "rocked" t-5's at your power levels...there are far....far more that have broken them with less. I don't disagree, how you drive can absolutely effect the trannys longevity. It won't however ensure that the tranny will be safe at higher torque levels. Because of the previously stated tranny weaknesses, powershifting with near stock power levels loads the tranny in such a way that it put's extreme stress right at the heart of the trannys...eh...weakness. Now when you increase your torque levels to an extreme amount you are able to stress the tranny much in the same way...even without powershifting.
It's the same reason Joe Shmoe takes his friends out for a ride in his basic h/c/i stang and punches it while in 3rd going up a hill. Kaboom goes his tranny and he's left wondering WTF just happened. Perahps not the best analogy, but you get the point.
If you take your tranny out now while it's still good...you can typically get 500+ towards a new tranny setup. That sure as hell makes the hurt much less. Or, you could do what I did and wait until it finally happens to you. No, I have never powershifted. No, in turbo form it has never seen the dragstrip. No, the car has never seen slicks. Yes, the tranny went kaboom rolling into 3rd without much effort.
As far as the stock clutch goes...your best to keep it in. I'd venture to say that even though you may think it's not slipping....it is! This is most likely keeping your t-5 amongst the living.