Wouldn't this particular topic be best answered by the dev's of said file systems and what, if any, fragmentation etc, could and can and will happen with the different arguments ie -m 0, etc.? All it is now between everyone discussing this here in a Gentoo list is more or less conjecture, at least what I've read from most posts. Wouldn't the people who make up, design and take care of the file systems in discussion be the best ones to simply ask?
Not meaning to 'end' the discussion, just curious as to why no one has gone to those who would know best and brought it up and seeked out the answer(s) from them instead of people who only can guesstimate here. So don't get mad, get glad, heh heh.