The I6-242 burned gas in 4WD configuration, especially with the addition of lift kits, winches, etc. At which point these Jeeps often exceeded 4,000-lbs.
In 2WD, at 3500# with driver (pair of '01 XJ's), 22 mpg hwy was our commmon number, and 18-19 around town. The durability was unexcelled.
As for being slow, they aren't. I haven't ever in a situation where I wished for more power. The throttle response is instantaneous which makes them very easy to drive.
A straight six, be it BMW, Chrysler Slant Six or any HD Class 8 diesel is flat hard to beat. You don't see V-configurations where a straight will work, either railroad, ship or power plant.
That said, I hope the new motor is good, but I would never expect it to do as well. The Buick 231-cid V-6's were awfully good, and I hope the same for the new Jeep power. I do like Jeeps, but see no real successor to the XJ-Cherokee where size and space, power and utility, were ideal.