Max alt on my aircraft is FL510, been there and you sit between the high and low speed cues. I've got no idea if the tcas can issue an RA at 510 (it probably can, it's not that clever) but if it did I would assume it was some lunatic at my level pointing at me and even if I could only climb by 2 or 3 hundred feet that should be enough to not hit him. You can't shove the nose down very hard at that level either so he would hopefully be nursing it down at the same time. But you've got to assume that the other aircraft isn't doing anything to avoid you. So we might not miss by much...but hey, it's a miss.. I know the airframes have been to 540 during testing so it's not going to do anything particularly horrible and I think the prime directive in all aviation should be "don't hit other planes" because it's really bad on the CV