When I studied with John Yeh on Eb clarinet, he was using a Vandoren 5RV. But the missing information is that John can play on anything - and make it sound and work great.
I was playing on a Vandoren 5RV at the time as well - and it was ok. John thought it was really great......?
Anyway - I later purchased a Greg Smith Eb made from a Zinner blank. By the way, Richard Hawkins was the maker of the mouthpiece that Greg sold me - serial number 004 (!). Richard used to make all of Greg's mouthpieces before they parted company a year or so after Greg started up.....but, I digress.
The Zinner based Eb is exceptional. When I learned how to make mouthpieces, I adjusted the Eb I own to play better. I've sold a large number of Eb mouthpieces - most which play better than the one I own - but I like the fact that I have this original Hawkins with Smith's name on it, so I keep it and use it! The bottom line is this - when it comes to Eb clarinet mouthpieces - the person making the mouthpiece had better be able to play Eb clarinet well - otherwise - it's a crap shoot. Having said that, an Eb mouthpiece made from a Zinner blank is, in my professional opinion, far beyond the Vandoren line.
Best wishes,
Roger Garrett