The presumption that replacing an S197 live axle with an IRS necessarily will improve handling by default is wrong, especially as well-sorted as many S197 models' live axles are, even without Watts linkages or performance suspension configurations like an FR-3 pack helping them out.
An S197 IRS swap surely will change handling properties but not necessarily make them better or worse. Above all, the handling will be different. S550 is a perfect example. When Ford borrowed a Fusion IRS to use as a development boilerplate, one of the engineers' first discoveries was how unsuitable it made the carryover S197 front suspension they originally paired it with. They didn't eventually redesign the front suspension just to refine its performance with an IRS. Ford scrapped the S197 front suspension altogether because pairing it with an IRS initially made S550 handle worse.
Whether the goal is to create the best all-around balance of handling properties for a daily commute and occasional performance driving or the best, most aggressive handling for track only use, setting up a car's handling rarely comes down to just swapping out one component and calling the job done, no matter how limited a particular component's potential may be or how much more a different (or different type of) component may deliver.
What matters most is how all of the various suspension components work together to deliver a particular set of handling characteristics. This is one reason why Ford Racing historically has offered "handling packs" which can include everything from springs and dampers to bars and bushings that work together predictably rather than single parts like lowering springs which may lower the CG and stiffen the ride without compensating for the different bounce and rebound and sway and stabilizer properties that make the car handle as well as possible rather than tweaking one or two characteristics without regard to how well the car handles overall.
This kind of holistic approach is how Ford engineers were able to make the BOSS seem so planted and nimble. There's no one magic bullet. It's how all the pieces work together. An that remains the best way to improve S197 handling rather than just ditching the live axle because it happens to be the one piece that draws the most attention for being the oldest type of component that's used.
Rather than swapping out suspension types, or even a simple set of springs, the best way to improve handling is first to decide upon the specific handling goals you want a suspension to achieve and then assemble the components that work together best to deliver that combination of characteristics. Because replacing an S197 live axle with an IRS also would require a front suspension redesign or using an aftermarket complete suspension solution that centers on an IRS with all the other components to make it perform as well as possible, doing the job right and actually improving the car's handling usually is a very expensive from-the-ground-up propositon.
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