My limited knowledge is that all AWD Magnas run an 18mm rear bar.

Body roll could be a few things, I'd start at tyre pressure and work my way up before shelling out for a rear bar.

I've upgraded the rear bar on my AWD to a 22mm bar from Mako - not exactly an inexpensive upgrade. Worth it? I think so, but I'm they guy who shelled out the cash. Car is sharper and understeer is reduced, steps out more if you're ham-fisted in the wet.