Need some help controlling body roll

A 255/60/15 should fit easily on a Duster if the backspace on the rim is correct. 275's should fit on most Duster's without rubbing when the proper backspace for the car is used.

The springs may be sagging a little, but the spring rate isn't effected and the factory spring rates in the back weren't that far off compared to the front. If you like the current ride height of the car and don't want to replace the rear rims with ones that have the proper offset your next option would be sway bars. If you have stock torsion bars up front don't just add a rear bar, as the car will end up in oversteer. Add front and rear sway bars, which will help with the overall body roll. That's probably the easiest fix, if more is required you'll either need new rear rims or to start upgrading torsion bars and springs.