Both of you guys are right IMHO. Underfire theoretically and Ogger practically.
It is clear that if MDA is 400ft (airplane is not safe below 400ft unless visual references are avail) than according to basic logic further descend in IMC is prohibited. If you convert 400ft MDA to 400ft DA the airplane will descend below 400ft. (underfire)
Its also clear that authorities can just say: OK, never mind MDA. We are granting you permission to descend below MDA. (Ogger)