You don't need ETOPS to fly across the Gulf of Mexico PHX-CUN, but you definitely need EOW.
EOW, or "extended overwater" applies to aircraft with life vests, life rafts and certain other water-related safety equipment installed - I think HF radios too? If an aircraft is not EOW approved it cannot go out to sea farther than, I think, 50 miles.
ETOPS aircraft are by definition EOW-approved, hence the US West ETOPS 757s can fly PHX-CUN overwater. If US East EOW Airbuses are placed on the route, there should be no problem with flights being lengthened. I don't believe any other America West aircraft are EOW-equipped.