I agree with the other guys you list, but I'm starting to wonder about myself. I don't seem to be able to make memorization stick that quickly. 12 cubes seems quite possible for me, and 13 doesn't seem out of reach, but it has taken me a very long time to get just a little faster, and I feel like I need to find some new thing to do to give me a quantum leap forward, or else I'm stuck in mediocrity. It was easy to get to 10 cubes in an hour, but then it got tougher for me.
I had 10/11 again last night, in 50:53.71 (34:58 memo). It's one of my best times ever for an 11 cube attempt. Two corners twisted - I memorized the wrong letter for one of the twisted corners.
One thing I do wonder about: I do almost all of my attempts at multi late at night (so my daughters won't disturb me - or the cubes - during the solves), when I'm usually quite exhausted. I wonder how big a difference it would make to be doing them earlier in the day? I know that my 10/10 in 45 minutes in competition was way faster than I had ever done them at home; perhaps it would make a big difference.