Absolutely... Swap out the front and rear sway bar end links and bushings with aftermarket universal polyurethane parts. Handling will drastically improve although you'll definately feel a stiffer ride. I was able to fabricate some backer plates and swap the front sway bar mounts with polyurethane universals as well but I'm not sure how they look on the '99. KYB produces front struts for the LeSabre but you probably have rear air struts that are only available from OE. If Monroe has a rear fitment for the rear air struts now days you'll find Monroe units are typically on the firm side. When it comes to brakes I have no idea. If the 97-99 bonneville had a 4 wheel disc option you can look into using those, bonneville and lesabre fitment should be almost identical. Bonneville dual exhaust from the SSEI should bolt right up as well, if you want a nifty exhaust upgrade go to the dealer and buy the SSEI system for the pontiac.
If you are mechanically inclined and you want to experiment, you can do a couple things in the steering. First of all replace all the U joints from the upper shaft down to the rack and pinion with new OE parts. If you can access the pre-load adjustment for the rack you can play with tweaking the pre-load but don't take that on unless you feel really comfortable with mechanical stuff. Over tightening the pre-load can cause the steering wheel to jam up and will lead to premature failure of the rack. When I adjusted mine on the 2004 I cut the wheel to extreme before adjusting because typically the rack is more drastically worn in the center. Adjusting pre-load to ensure proper operation at the extreme end will virtually gauruntee a tighter feel on-center while still maintaining proper clearance.
An easy steering upgrade is to simply replace the outter tie rods with Moog brand units. You should feel a big improvement if you have older tie rods, the outters tend to wear out long before the inners. When inner tie rods are loose its probably a good idea to just buy a reman rack. Don't forget to have the car professionally aligned after any steering work, especially tie rod replacement. Follow all factory or service parts guides when it comes to fastener torque and use a well-calibrated torque wrench on critical steering components.