I've got IPD 'track spec' swaybars in my 850R wagon. They came in the car so I can't comment on before or after but I did have an 850T5 sedan fitted with standard bars and I find the R is far less prone to push understeer when attacking bends. It also feels to hug corners and roundabouts way better. To the point where I can keep accelerating and I have to be careful not to run into the inside edge of the corner because it just wants to keep hugging the inside line. It also feels more direct on direction changes. The rear end also feels much more like it's tracking with the front of the car, rather than sort of reacting to whatever the front is doing, if you know what I mean.
The T5 was fitted with coilovers and the R with Bilsteins and stock springs though. Also delrin subframe bushings in the R. Pilot sport 4s on the R and lesser tyres on the T5.. So that throws in a few variables.
If you want to tighten up the response and feel though I would recommend. But I'd suggest decent tyres also help alot.