I really doubt that using -fnowrapv or the equivalent provides much of a performance boost in practice.
Most of the examples I've seen have been of the form "aha! I can (incorrectly) assume that this if statement that the programmer put here is a no-op!" In all of these cases I've seen, the "optimization" is something that a good programmer could have and probably would have done manually anyway.
I'd really like to see some real-world performance numbers about the effects of this optimization. Based on everything I've seen so far, this is a case where compiler writers got so carried away considering what they _could_ do, that they didn't stop to think if they _should_.