Properly matching your upgrades, even if you only do one at a time, will give you a car that handles better, safely.
Quoted for emphasis.
Also keep in mind that swaybars are a
fine tuning 'band aid' of sorts. Ideally, with properly rated springs and matched valving dampners for the application in question, we wouldn't need swaybars, or at least a very small one to get that last ~10% out of the suspension.
IPD sways are fantastic for stock springs & dampners, and great with mildly uprated springs (i.e. IPD, B&G, Jamex, etc.) and "off the shelf" sport dampners (i.e. Koni Yellow, Bilstein B8, etc), but still a band aid. Off the shelf solutions are great for ~95% of the masses, mostly because the cars being fitted with these parts are never fully, or properly utilized to their potential. For a DD, IPD suspension bits really transform a stock Volvo into something quite fun. In dedicated motorsports applications, chances are smaller bars are being fitted while custom rate springs and matched dampners are taking the main stage.
Big bars are fun, but they're also [typically solid and thusly] heavy and that added
unspring mass mass can have its' own added difficulties with handling prowess. Dual small(er) bars are also heavy, and add more points of failure to the overall setup. Still fun though!
A lot of it also boils down to the driver and their abilities as well. A crappy driver still can't wring the performance out of a car that a good driver can, even when they're both sitting in the same prepped car.
Just my $0.02, flame on...