"I put 285/70/17 on my JK with 6 speed manual and 3.73 gearing. It was a noticable change in power going up hills. Have to downshift to 5th more often. Other than that not much drivability difference."
I only get to use 6th on the highway occasionally as it is, now. 5th gear is rarely over 3k RPM for me, unless I'm going 80 in Utah.
I might get away without new rims, but it's not clear whether the bigger diameter will rub or not. I'd prefer new rims instead of spacers, if there is an issue with rubbing.