I wasn't suggesting ignoring Crit, but you get more than arp from Con, you also get Hit Points, with Dex you only really get crit. If you had more points into Con, Constitution Focus is worth considering. Basically with a Con focus you can ignore Defense(which I feel is a horrible stat for Instigators) and Steely Defense…
With the Constitution Buffs it seems to me that you may get more out of Con then you do Dex. Also wouldn't you be better off with less points into Steely Defense and having less or no defense (it reduces your determination gain) and replacing this with Hit Points this way you will be able to build up more determination…