In regards to monette mouthpieces and gap:
I found that their standard mouthpieces (the ones made for non-monette horns) are deliberately made to have an enormous gap. I think this in order to compensate for how open the mouthpieces are, and to balance those characteristics out with respect to a trumpet that is built for mouthpieces that have pretty extreme geometrical differences from a monette. I found some horns that work great and some that don't.
The gap matters, but is only part of the package. Things like throat, cup-volume, backbore, and horn attributes also contribute. I would say that based on my mixed results in a variety of horns with monette and non monette mouthpieces (see below), that the gap play the smallest part in overall balance. I would strive to gain balance with the other mouthpiece/trumpet attributes as I think they will change the feel the most. Then, when I have all that in a good place, the gap might be the final thing to adjust. Otherwise I'll keep second guessing the gap when other parts are more of an issue.
On my Kanstul 1600:
The B4S and BL worked great. I had no issues and loved the match up. It was when I had this horn that I has first hipped to adjustable gap receivers by Ken Larson (who was my teacher at Interlochen at the time). I can't really comment on this match-up. I can only really say that it worked and my teacher thought it worked well enough to let me keep the setup.
On my Austinwinds LT:
The B2S3 and B2L that I had switched to also seemed to work very well. Granted, I had to have them take off their special lead-pipe and put a more standard one on the horn in order for the standard weight monette mouthpiece to fit in the horn. Also granted that this is still a Kanstul horn. I think the larger bore of this horn complemented the monette mouthpieces better, made it more balanced. However, I did notice that the horn really only wanted to play lead or jazz. Getting a focused and compact classical sound was way too much work. I eventually switched to a GR 67 series mouthpiece on this horn. It zipped around much better. I think the gap may have played a part in this.
On my Larson GFTLX:
This horn was build with a Laskey mouthpiece shank in mind. Therefore it was no surprise that the B2S3 was not optimal. I think this is a classic example of a horn that does not gel with a monette mouthpiece. Be it the gap or something else. This was the best playing horn I ever owned. However, the investment it represented had to be evened out in order for me to get all the trumpets I would need for school :( I'll get a whole set when I make it big time!
On my Yamaha Vizzutti horn:
It worked fine with this horn. This was a very tight horn to begin with and so the open mouthpiece helped make it more suitable for non-classical settings. A horn that is this well made and consistent with the level of efficiency that comes from the tightness is pretty much guaranteed to play even and great with any mouthpiece. My only qualm with this horn is how stiff it felt when improvising, and how piccolo-like (read: ultra bright and piercing) the upper register got when playing lead. I needed a fatter lead sound and was willing to give up some efficiency for that sound.
On my Taylor:
Had to have the mouthpiece receiver opened up to make the gap smaller. Major endurance/efficiency issues with monettes on this horn. I think the design philosophy between Taylor and Monette are two ends of the spectrum. I could not make it through one set of casual jazz without being destroyed from trying to play with this setup. To be fair, nothing really worked that well on this horn other than a Taylor mouthpiece and a Giddings and Webster mouthpiece I tried. This was a great horn though. It was agile like a cornet, could sound like a flugelhorn, and could scream on westside story or in big band.
On my Yamaha 8340EM:
Same as the other yamaha, but this horn had a giant bell and was a little more balanced. This horns issue was how light it was. The monette mouthpiece is basically 1/3 the weight of the horn! I would need an XLT weight blank to make it work. Also, this horn has so little resistance that I ran into efficiency issues with a monette. With a non-monette this horn is great if you like ultra-lightweight horns.
On my Flip Oakes Wild thing (current Bb):
The monette workes great on this horn. I suspect that the design philosiphy of this horn is closer to a Dave Monette's than anyone will admit. (and that's not a bad thing by any measure). The WT horns may be the non-monette-priced monette-like horns out there. I do notice that with a regular mouthpiece the horn plays a little lower on the pitch. With a monette its pretty high on the pitch. I don't know why that is. Probably operator error.