If you want to be able to do 80 then go with 31x10.5R15's. You can run those with no-lift with a little bit of rubbing or add a little lift.
You're fighting two wars with taller tires. One has to do with the tires themselves. You're changing the overall gear ratio by going larger. Larger tires are heaver and will make the Jeep feel a bit more sluggish so you'll get reduced acceleration as well as braking. For every pound, it's equal to 2 pounds of sprung weight inside the Jeep. So a 5lbs increase in tires will be equal to about 40lbs of additional weight. There is also aerodynamics as a smaller tire cuts through the wind better. Larger tire also means a larger contact patch which can be more friction with the road.
On the second front, larger tires typically means lift. More lift will cut down on the already lacking aerodynamics. The isn't a problem till you reach about 35MPH. The faster you go the more wind plays into it and it's exponential.
I've driven 5 years with 32's and stock gears and when the National HWY speed was 55 MPH, it wasn't too bad. I used 5th occasionally as well. Now that it's 65MPH with most people doing 70-75, it won't cut it. You would want to definitely regear for anything larger then 31's and even for 31's you still light want to regear.
You might also want to check out the Skinny tire thread. There are pictures as well as my ramblings on skinny tires. This is after 10 years or so running 35x12.5R15's and having a daily driver.
