I have a Supercharged l405 running on Pirelli 275/45 R21's. I shoot and want to go offroad and am thinking of getting a second set of wheels R20. The only AT tyres size i can find is 275/55R20 but the spec suggest 255/55R20.

Will this be a problem?

 

You could run in to clearance issues with the additional height of the tire.

If you have a GAP tool you should be able to add a touch of height to take care of the issue