I've been wondering about that myself, and I too would like to know what the experts think. Would 3 fists of iron, followed by the earth finisher (+2 crit mult) on an auto-crit mob be a good idea and/or is it worth spending the 1 AP for those situations?
I'd consider testing it myself, but I'm not yet at a point where I can reliably stun anything that will live long enough.
P.S.: Do burst/thundering/maiming effects deal increased damage accordingly to the higher crit multiplier?