The rear swap bar wasn't factory equipment on every XJ. Lots were sold without.
I noticed a positive difference in handling when I put one on my XJ. Not a huge one but it was there.
The front sway bar, well, I sure as hell wouldn't take mine off again. I ran without it for a little while and the car just wouldn't change direction fast enough for my liking. Sure, it was driveable. At 70 mph, though, try and juke the wheel - as if, say, to avoid another motorist doing something silly - and watch how freaking little happens. It just didn't seem to steer the same without that sway bar.
Rode smoother, though, across uneven bumps. + Chicks dig body roll.