I've been using a Bach 5G with my Bach 42 for several years, but I've struggled with my high register. About two weeks ago I tried switching to a Hammond Design 10M mouthpiece with my 42 because I switched from second to third in a community band I'm in, and it has had a surprising effect on my *high* range that I did not anticipate. I now find it easier to play notes in the 7th and 8th partials, where I used to struggle. It's still a work in progress, but I feel like I've made a jump in ability. Perhaps the wider rim diameter of the 10M is helping with that? I do have pretty full lips, but it's disorienting to think that a rim diameter that large would be a benefit with my high register. At the same time, I'm trying to really focus on good breath support in my high (and low) range, and I do notice that the larger 10M requires more support, but it is really making me think that I should stick with the 10M for several months and build up my conditioning and high range with that, rather than transfer that approach back to my 5G.

Yes, for many (or most) players, the "conventional wisdom" that says smaller=high range is just plain wrong. In many cases it inhibits overall function and limits high range.