I still have mf9 firmware on my phone. I haven't updated it to mk2. I haven't had any issues. I am running a dual boot of sacs custom 5.2 touchwiz and the latest slimkat aosp weekly rom.
So I guess to answer, they should be compatible either way. Maybe not with pre mf9 firmware but I can't confirm that. I haven't really found a reason yet to update to mk2 modem but I've only owned this phone for 3 days.
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