First, I say fix the Mossi FB Mk VI. I believe the case has been made very thoroughly and with plenty of evidence for bumping the top speed up 15mph or so across all alt levels.
Then, I say add the Mossi B Mk XVI.
I think allowing the best bomber version of the Mossi the ability to show what it really can do with the high speed and good medium bomber payload (3-4K), AND correcting the FB Mk VI speed would give this legendary plane its rightful place in the AHII arena instead of looked upon as an easy kill in any sense.
I dont think we need the Mk XVIII.

The other ground pounder planes with large guns dont seem to be worth their while (Hurri IID, B25, etc) and a Mossi with a 57mm AT cannon made for anti-shipping isnt going to fare any better.