Well, I did this on my Z71 Tahoe and there was a change. Went from 17s to 20s for summer and yeah there was a slight difference. The biggest change that I noticed though was in braking and handling---more rotating mass to have to stop and unsprung weight in the corners. The Jeep should be no different. But ading larger wheels is going to be more drag and weight for the the engine to deal with, so there'll be some change for sure. The experienced folks here will have to comment with their real world findings.
My guess might be 2 to 3 MPG...a total guess.