Hornady reccommends 1:7.5 because of the loong Match bullets the caliber was designed for. IF you want to MAKE this caliber work for hunting in general and Coyotes, because it's compatible with AR-15 platform, then the 1:8 actually is a better fit ( still over twisted for any 6mm hunting bullet made). Most .243's have a 1:9.25 twist and are capable of shooting 100gr bullets beautifully.
I have a 1:8 BCA .243 AR-10 and it shoots 58-87 vmaxs as well as 100gr factory deer ammo in itty bitty groups. I have no idea how it shoots anything heavier.
Biggest dillemma with 6mm arc is the long throat for the long heavy bullets. Lotta jump if using shorter regular bullets. Can only load bullets so long and keep neck tension.
My guess is that a 6mm ARC will shoot best with the longer varmint bullets for coyotes, but will be slow with anything above the 55-58 grn bullets.
Just a mismatch of a caliber for coyotes IMHO. Would be better off with an AR-10 .243 and never have to worry about the when the 6mm ARC trend dies off.