I run the 084 in both machines, over 1200kms so far on each belt and still like new. I got 600km outta my first 083 before she blew, but deflection and alignment was way outta wack from factory so surprised i got that long outta it on a '12.
I sucked it up and got a 4 pack of 084 belts from Country-Cat in the states awhile back when the dollar was better so still have 2 spares on hand.
Dayco xtx 5046 is on mine waiting to be broke in. Ran Dayco on my 200hp m1000, didn't blow a belt or ever use the 1 Year warranty, so will try on the m8 now