Even tiny tiny CPUs can do sub in one cycle, so I doubt that. On super-scalar CPUs xor and sub are normally issued to the same execution units so it wouldn't make a difference there either.
On superscalars running xor trick as is would be significantly slower because it implies a data dependency where there isn't one. But all OOO x86's optimize it away internally.
Even tiny tiny CPUs can do sub in one cycle, so I doubt that. On super-scalar CPUs xor and sub are normally issued to the same execution units so it wouldn't make a difference there either.