Yes You'd be better with a Q-Lock or similiar aftermarket feedneck on the Ion though, as the stock one is pretty crap and a clamping feed would be adjustable for a better fit.
yes it will fit, but be careful, the first time i took my revvie out of my feedneck i cross threaded my breach assembly (front fire chamber), i've had to replace that now and i've invested in a q lock and life is much easier now.
I got a redz clamping feedneck for mine. saying that, there isnt much difference between that and the Q-lock in use-ability, they just look a little different.