The manual downshift mode you described causes the same issue as flooring it in auto mode in higher gear as mentioned. I tried it a few weeks back and actually threw a DCT overheat code quicker than I did when I used to punch it in auto mode.
I totally understand your feeling of wanting to go WOT on every on ramp. This was REALLY easy to do in an NA M3. On a s/c M3 no matter 500whp or 600whp, you need quite a lot of room on the highway to be redlining. Your car will be at 120+ in the same time you would normally hit 90 say taking off from 60ish. This is a HUGE difference on the highway. 90 is a little above flow of traffic. 120 is flying by traffic. I'm sure someone can break down the exact numbers to see what speed a stock M3 would be at compared to a blown M3 if they both took off at 50-60 for x amount of seconds. I'm just telling you a rough feel that I used to notice myself getting up to 90 on a brief WOT on ramp situation that now has become 120-130+. It is insane. I didn't imagine it being this fast and me not being able to go WOT redline every on ramp like I used to. Coming from a guy that used to use auto mode WOT quite often in my NA M3, you will not miss the manual drop down. Even if the feature worked without any clutch problems, I doubt I'd be using it much. Just drive a s/c M3. There has to be a ton of members around you. I'd you are in NJ for any reason come take mine for a spin. Just don't try the drop down change