Not quite. A collision found part way thru the stack still allows you to generate collisions across the entire stack, because you can skip the inner-most steps.

e.g. if we have
crc32(sha256(y)) = crc32(someValue)

if crc32(someValue) collides with crc32(anotherValue), then there is essentially a full stack collision, because you can literally ignore the innermost sha256() function when generating your X11() collisions.

This is partly related to the nature of using a hashing algorithm for PoW - it's unnecessary to have a collision attack at the inputs, only somewhere along the hashing chain before the output.

Of course, this is all mainly about future-proofing - but in the long-term, I think this may end up being highly significant.

Heh, I'll up the ante and bid $0.02.