I want to first congratulate the op! You found the car you wanted, and it rides the way you want it.
Self leveling suspension is a complementary system, to the rear standard spring/strut suspension, which is intended to Maintain the ride height as best possible. That means the x5 is allowed to feel the same way in terms of handling with or without additional weight.
Now this notion of why the car rides differently is difficult. The original vehicle was a front heavy diesel, SA PACK, 20inch wheels and the new vehicle is a lighter 5.0, 20s newer. There way too many variables in play to specifically attribute the self leveling to better handling.
BMW and Mercedes Build Beta Products!
Lexus build Release Candidate Products!
Why does this matter?
CAUSE EVERY TIME someone says Lexus has better build quality, one should consider what Toyota their Lexus Engine and chassis platform comes out of and how long that part has been tested in the real world!