I like the sound of the R spring option if you can find a wrecked one.
How about just stick the oem's back in and try the ipd sway bars? (I know I've mentioned it before, they have me excited) Gee the reviews are good! I recommend giving them a read..
https://www.ipdusa.com/products/4892/109280-front-rear-anti-sway-bar-kit-xc90
example:
"Absolutely amazing. Every aspect of driving the car much improved. Handling in curves of course much better. But also straightline highway driving, steering feel, windy conditions, lack of body roll, even just turning into driveways at low speed you can feel the difference. As far as I am concerned its the best mod you can make for the money."