Sorry to bring this old thread back to life, but I found this other old thread on Amiga.org that seems to suggest that, at least in the distant past (back in 2008 when all this Amiga hardware resurrection fever started!) the Minimig CPU worked fine at 14MHz:http://www.amiga.org/forums/archive/index.php/t-47510.html
Just to refresh your memory, 14MHz would be a very good intermediate speed to have set as Minimig's default (after the real, original 7MHz speed of course) because it would avoid slowdowns in games AND it wouldn't cause music notes to be skipped since A1200 CPU ran at that speed and an A1200 doesn't show any skipped notes in games (Monkey, Loom, Ishar...).
Now that minimig_emu has published his AWESOME new arm firmware, we could have NORMAL (7MHz), FAST (14MHz) and TURBO (50MHz) cpu speeds.
Maybe there's info on that thread regarding why 14MHz doesn't work anymore?