My RC 4.5" was enough to run BFG MT 32x11.5x15 no trimming. Tight turns required minor trimming.

Two factors. What is the actual measued lift of the kit you choose, and what is the actual measured diameter of the tire.

If all things are equal, 31" tire is 1" shorter, 1/2" per side. So the lift could be 1/2" shorter, or 4", like BlueXJ said.