Although I don't have a Mirafone 187, I have noticed a large impact from the backbore of the various mouthpieces that I have tried. I tried a Doug Elliot R cup with a 4, 5, 6, and 7 backbore. For me the 4 & 5 felt stuffy, while the 6 felt good and the 7 was too hard to control, all on the same horn.
I noticed the same thing when I switched to the Denis Wick 3L that came with my Besson 983, vs the Helleberg 7B that I had tried at first. They have similar sized cups, but the DW has a larger backbore and made the horn feel much more open.