From what I read, the firmware updates for higher CPU frequencies never came. Phase5 dropped support of the idea entirely, since Motorola never released 66 or 80MHz (full) '060s. Apparently, the MKII firmware author (Carsten Schlote) left Phase5 shortly after the MKII was released which might have had some impact as well. (The above might be inaccurate, it's just what I read perusing old usenet posts.) Mr. Schlote released an unofficial firmware, but I never tried it because (1), it's AFAIK irreversible, and (2), as you mentioned, fear of a b0rked MKII. I read a few stories concerning the latter. :-/
I think you are right, it shouldn't damage anything to try. I'm guessing the firmware update would have relaxed some RAM timings, which explains why good RAM is required for 80 (and even 66MHz) overclock and the board jumpered for '060 operation. (I don't think the SMD jumpers are of any help here, unlike the Apollos.)
