I broke the blocker rings in my t45 powershifting my 00 gt on 1-2 shift. I took the trans out myself and took the trans to a shop it was $800 to fix the blocker rings and 1-2 sychros. I replaced my pilot and throwout bearing and put in a new clutch at the time. Total cost was about $1200. I orginally learned to powershift on a v6 stang which was perfect because not too much power but I also learned u can break stuff pretty quickly. If you want to learn how to do it I recommend 3-4 shift that is the easiest to learn 2-3 is tough
As for shifting without a clutch... yes, you can at LIGHT loads, but obviously if there is any resistance, you aren't matching the revs well and just killing your syncros. If you are smooth enough, you can slip it out of gear when you let off the gas and slip it back in the next gear at just the right rpm. But it isn't easy on a 4.6 because of the hanging idle. More of a lazyness trick really.
Yeah, the hanging RPM on a 4.6 make this technique very difficult to use - upshifting, by the time the RPM have finally dropped, the car has coasted down too much and you missed your chance to get it in to gear. Downshifting is a little better but if you overshoot then you're stuck waiting for the revs to drop again.
This was really easy to do in my old VW Fox, I could drive all day without all day without ever using the clutch unless I was absolutely forced to come to a complete stop and didn't have any downhill incline at all. Not that I recommend driving like that, of course, but that skill has come in handy a couple of times and if you're matching revs properly, it's fairly harmless. If you aren't doing it right, you can wear out your synchros in a hurry. Of course, powershifting isn't exactly good for your synchros either.
A friend had a Mazda 323, maybe an '89 or so - it was the easiest transmission to powershift on ever. All you had to do was just pretend like you were going to step on the clutch and, BAM! it was already in the next gear.
If you depend on this car or don't have spare cash to throw at transmission repairs, then don't do it. Some tenths aren't worth it.
another trick for learning is half throttle power shifting, give it 1/2 throttle, then once it gets to redline shift with it at 1/2 throttle, once your good at that bump it to 3/4, ect.ect