It is said, in racing circles, that removing 100lbs is about the same as gaining 10Hp. So that will give you an idea of how much weight would need to be removed. Basically, the entire interior save for the driver's seat and that would need to be changed out for alight weight racing bucket. All of that would weigh in at what, maybe a tad over 100lbs?
Really can't see any good come of this.
IMO, for mileage, run the narrowest tire you can find to reduce the rolling resistance, lower the Heep to reduce it's brick like aerodynamics and utilize "hypermiling" driving techniques. Shutting the engine down at stop lights is a good example of hypermiling. If you look at the curent crop of cars that are rated to get high MPG, a number of them shut down at the lights.
But, at the end of the day, it will still be an XJ. Not exactly the world's best example of fuel efficiency. I get a tad under 17 in town and near 20 on the road and am well pleased to be getting that. My heep is nearly 6" up from stock and I run P285/75R16 with 4.56 gears. Can't expect mileage out of a lifted rig.