I do the same in text sometimes, but there's more likelihood of confusion when used verbally unless your accent uses something like "Zed". In most American dialects "C" and "Z" are sometimes hard to distinguish verbally. You start into "C as in cow" and "Z as in zoo" games at which point it is easier to revert back to "authn" and "authz".

Never thought about abbreviating verbally and I never heard anyone doing so, but maybe that's because in Dutch (much like in German) we're used to longer words :D