Huh, can someone list what we know for sure that Shanks and Mihawk can do, and then list things we presume they can do?
Mihawk can:
-Outskill and hit harder than all known swordsmen, casually delivering "world's strongest" slashes large enough to tear apart icebergs, with one handed swings(imo he beats Shanks because 2 handed slashes > one handed slashes)
-Keep up with Shanks in speed
-outlift Zoro
-Get through the marineford war unscathed and unaffected
-Create Black Blades
-Use Haki better than Return to Sabaody Zoro's
-Throw an opponent off-balance by devastating the landscape
-Do every feat he's ever done without going all out, except maybe the latter part of Marineford.
Mihawk might be able to:
-Use top-tier Haki
-Use a special type of skill we haven't seen yet
Shanks can:
-Oneshot Kid while cutting through his attack, despite Killer jumping in between them and getting oneshotted as well.
-Use the best of every Haki ever shown in the series
-Completely negate a punch from Akainu
-Clash with Oldbeard on his home turf
-Set his sword on fire somehow
Shanks might be able to:
-Use even better Haki
-Use some other kind of power