Anyone know if the Shires 5G has any significant difference to the standard Bach 5G? It came with my series and for some reason it feels SO much bigger than the Bach 5G I use. It feels almost like a small bass bone mp.

Your lips are not wrong. The Shires 5G is indeed larger than a typical Bach 5G.

This represents a good reason to abandon the "Bach-equivalent" nonsense of mouthpiece sizing. Bach's nomenclature never made sense to me. Nor does Shires/Yamaha. Or almost anyone else. [Greg Black? Come on - 1-3/16? What the heck does that mean?] If Cup I.D. is such an important parameter, why not label mouthpieces with a name or number that refers directly <I>and clearly</I> to that measurement (as do Doug Elliott, AR Resonance, Laskey, and a few others)? Or use a sensibly organized naming system (as Josef Klier has attempted)?

And why not use another symbol / letter to refer to another significant variable (Cup shape / dimension) - again, like Doug Elliott or Josef Klier?

I have a Shires 4G which came with my Shires large bore 8 years ago, and it's pretty big. It feels similar to my Doug Elliott XT104/G+ cup. The 104 rim is similar to a Bach 3G diameter.

Just try to make the Bach numbers compare to the Marcinckiewicz mps that I play. Marck's measurements are all different as well.

I play a UMI/CBK-- same mp under different names, I have one of each-- the ones that are tulip shaped like Denis Wick's. The 5G is deeper than a Bach with a slightly wider rim. It's the only non-Marck that I play and works well on my euphonium.