The 6 runs about 190hp, 235tq. The 4banger is about 147hp, 165tq. Gas mileage is near identical on both. Any amount of lift or increase in tire size will have atleast some effect on gas mileage...most stock Wranglerss are lucky to get 20mpg though. Gears, rebuild kit, and install will run you atleast $1k, but is recommended on both 4 and 6 models with 33+ tires. It's liveable without (as I've been doing the past year) but sluggish.

On my Jeep, I have a ram air intake and throttle body spacer....and notice little to no difference. I won't even bother with exhaust, being the only noticable gain you'll see is in sound. I had the cat removed, but replaced it due to even worse gas mileage. If you're buying a Jeep to go fast...it's the wrong vehicle for you. I hear about people with 4bangers and 120-150k+ miles quite often...like I said, it all depends on how much you abuse it and how well you maintain it. Most Jeeps take quite a beating, so with that in mind, I'd say they're pretty dependable.