The OE 255/40R21 XL 102 tire is a performance tire size with limited selection, most of which are made from softer, fast wearing compounds (no tread warranty or 20-50K), at very high cost per tire (~$300-500 each). The more practical 235/60R18 size offers 6X the selection, including lots of options from 50-90K mile tread-life warranties, and the tires are 1/2 the price on average.
My experience with wide 40 series XL performance all-season tires is fantastic handling when measured in terms of holding G’s in a corner and hard braking or both combined but not particularly good at anything else. 40 Series XL has very stiff sidewalls and calls for more pressure for the same load carrying. This results in a ride quality comparable to an unloaded HD pickup on E-load range tires inflated to 65PSI. Every little bump in the road is jarring, loud, and abusive to the occupants of the car. If you’re planning to take your XC60 to a track and see what it can do, sure, keep the 40 series on it… If you’re planning to drive your XC60 out in the real world, filled with potholes, cracks, crack seals, driveway curb transitions, manholes, bridge expansion joints, rail road crossings, frost heaves, washboards, drainage swales, and other misadventures in road imperfections, you might want a tire/wheel combination designed to absorb those things… Your ball joints, control arm bushings, shocks, wheels, lower back, and ears will all appreciate it.
XL tires and big rims are heavy, this results a more steering wheel force/input to change the direction of the steering quickly, so while they can technically hold a corner at higher G’s before letting loose, they don’t feel as nimble in everyday driving with a couple fingers on the steering wheel. The additional weight, combined with the more aggressive wide/short tread engagement with the road, comes at the cost of acceleration and fuel economy as well. The wide contact patch is also harder to steer in parking lots at low speed or stopped. You can feel it placing more strain on the steering system.
The OE (29″ diameter) tire/wheel setup with 21″ rims is about 64lbs worth of wheel package. That’s about the same weight as a set of heavy duty, offroad ready, all-terrain 30-32″ tires on 16-17″ rims for an SUV or light truck. I would figure out if the optional 18″ volvo rims will fit that car and downsize. Go with 235/60R18 SL 103 and look for something lightweight both on the rims and tires. There’s opportunity here to save ~6-10lb per rim, and ~4-7lb per tire here. You could conceivably shed nearly 40-70lbs of unsprung rotating mass on your vehicle, while adding almost 40% more sidewall, and the option to reduce tire pressure to ~30PSI if desired. This will result in a significantly smoother, quieter, more relaxed ride, that will actually feel more nimble in real world driving as you don’t have to bear down on the steering wheel and be swerving around every imperfection in the road like a maniac to prevent a blowout or rim damage. You can just cruise right over all that stuff and enjoy the ride, and turn swiftly with 2 fingers on the steering wheel because there’s so much less unsprung weight to point and less bump steer and torque steer effects to fight. The Bridgestone Weatherpeak in 235/60R18 weighs only 25lb… It’s not a track tire but it will go in any weather conditions pretty safely, and rides smooth, quiet, and efficiently over the real world.