AND...my opinion is that the AEA and others will beat up on the FAA until they see the error of their ways....my opinion is also the the FAA is now feeling the pain of having a unique American only system that is internally incompatable with each other...the transponder requirment is for UAT because it is NOT a transponder, where ES is!
Personally the STC path is a bit odd for avionics fitment...understand it for any device that actually manipulates or modifies the airframe...but a radio device?