Ricardo,
With Extreme you doesn't violate the warranty I believe, but it is stated in several places that they don't guarantee if 3rd pty will work. So IMO if the issue will be related to an optic link with 3rd pty transceiver, GTAC will probably ask you to try on a different (original) transceiver as they have no capability nor reason to debug 3rd pty hardware.
HTH,
Tomasz