Hi, I have a 97 LeSabre, base (Custom) model. I have spent a lot of time making this car handle, and have done lots of research and hands-on investigation. Here'* the difference between the regular LeSabre and the Gran Touring package Lesabre, in the 92-99 range (may or may not benefit you if you are looking at 2000+, but GM sure doesn't change very much in the way of design when you're talking about Buick)
the Gran Touring Package ADDS
-->32mm front and 21 mm rear sway bars (stock Lesabre have 28mm front, 19mm rear)
-->Rear ALC suspension (standard LeSabre does not come with this)
-->high ratio transaxle
For the 92-99, that'* it. The Gran Touring is as much of a super floater ride as the standard suspension, but with slightly flatter cornering and the marginally stiffer rear (air assist) struts. You will still out-corner this car with just about anything on the road. I upgraded my base Lesabre with KYB struts (rock hard firm damping) and the Gran touring sway bars, and completely eliminated the sea-sick floaty ride. Again, the 2000+ may have benefitted from some redesigning; this I can't tell you from experience. I posted this info on the older gen. lesabres just because I doubt very many people have investigated the actual difference in LeSabre suspension packages. Even dealers couldn't give me ANY info on the difference part-to-part from a Gran touring to a standard suspension in my year. Even if a dealer has the part #* for optional equip, they usually won't tell you which option package is which.
Okay, so here'* what'* going to really answer your question.
Do 2000+ Bonneville SE'* come equipped with the electronic front struts (I don't know the 2000+ generation much at all)? If so, the LeSabres most likely do not have these, and will handle significantly softer and floatier. For comparison, 1997 LeSabre (custom AND Gran touring ) have the same part # front coil springs and strut cartridges as a Bonneville without the electronic struts. Both this Bonne and LeSabre will be super float sea boats. BUT the 1997 Bonne SE, equipped with electronic struts, does not have anywhere near as soft a ride as the 97 LeSabre (with the same coil springs). You're really not going to know how the LeSabre handles "vs" a Bonne SE until you DRIVE ONE. Even though the suspension components may be "nearly" the same, one or two small different part #* for coils, struts, different size sway bars, etc, will make a HUGE difference in how soft the ride is and how flat the car corners.