Mine is .728" or 18.5mm. At least that's the dimension listed on the MW website. I would say the horn is "ideal" regarding resistance.
Nice to see you hear again
I've played a 4V MW Eb cimbasso very similar to yours; to me the resistance is very similar to the resistance on my Haag, despite the bore being 2.5mm bigger on the MW.
Maybe its the different valves used, but I think the advantage of the Hagmann valves on a cimbasso is that it allows a more appropriate bore (IMHO) without making the instrument stuffy.