How much of your ride do you spend cranked over enough for that to make a difference?
I dunno, and as you say berms alter the dynamics some what, but you’re still not perpendicular to the berm, you’re just finding the point where the wheels are brdeline between sliding and gripping as the angle between them and the ground changes.
I did once stick a go-pro on my rear dropout and was quite surprised how close it gets to the ground in even relatively mild corners.
Either way, I’d guess that the controlled flex of a frame/fork would be far more preferable to a wheel flexing which would do all sorts of wierd stuff (it would have to flex and rebound as it rotated), so for wheels I hypothesise stiffer = better, for frames, less so.