I had it done. It was pricey. Look for a sale from your local dealer and then m are sure to use the 30% discount coupon from Volvo direct. Shifting is smoother now (wasn't bad before, but there is a good difference). Torque is definitely better. I feel I can drag race people now. I have no problem passing slow annoying drivers or getting past a truck or merging onto the highway and being immediately up to speed. If you can get it for between $899 and $945 go ahead and do it. Gas mileage seems to be the same for me. I expected it to fall but it has not. Car still is not a year old yet (2020 XC40 bought via OSD but not delivered until May of 2020).
I have installed it a couple month ago, and can share some impressions. My car is 2 years old, and I was sitting on the fence so long. It cost me around $1100-1200 US (with taxes and 15% discount). In Comfort mode it's revs more, but somehow smoother, and I don't feel much power added after upgrade (3hp). In Polestar mode..., I don't know, less jerking, I think. No fuel consumption difference, nor less, neither more. The car always was very quick, depend of your willing to hit the gas. Is it worth the price - more than $1000 US, I don't think so (IMHO). I've used my year-end bonus, so I'm not regretting it. But I didn't like the white badge, I like blue one I had on my C30.
I had Polestar installed, as part of the purchase, in my '20 RDesign. Most of the time I run in Comfort mode. When I punch the throttle from a standing start (Stoplight TT racing - ha!), there's plenty of punch.
Polestar changes shift points, throttle response, and mid-range torque. In pushing hard in the twisties, with Polestar and manual shifting, it's not hard to keep the R's up where the motor pulls like a freight train. As long as I'm charging, the changes in the tranny action is pretty good. What doesn't happen is effective engine breaking. Backing off to tweak entry speed while, for example, in 3rd with the engine turning 5K, the tranny doesn't drop down for almost a second - Not Good. I'm still working on how to manage that issue. By comparison, keeping the engine wound up without Polestar takes more effort.
I'm surprised to hear mileage is unchanged. Idle speed is up, shift points (in full auto) are higher. It takes gas to do that. There's another contributor to lower mileage: turn Polestar on and there's more inclination to get serious about going somewhere. Frankly, racing, even if only the fun of throwing the car around, isn't a place where mileage matters.
Polestar, as the default drive mode, is IMHO not the best idea I've seen. Would I buy it again, if it wasn't bundled into the sale package? Probably not. I use it under 10% of the time, and only when I'm pushing the car hard. Does it matter hitting an on-ramp, quick passing? Not really. BTDT, wasn't overwhelmed - it's not like stepping up to nitrous.
I think the mileage hasn't changed because, just like you, my Polestar is not engaged all of the time. So, the mixing of Comfort Mode and Polestar Mode evens it all out. If it remained in Comfort Mode, you'd find yourself stepping down more on the accelerator to make the car go as fast as you want. To merge into traffic on the highway or pass cars on the highway, etc. This is in automatic mode. I can't comment on manual shifting and what impact that may or may not have. Regardless, the Volvo XC40 isn't a fuel efficient car either way. No one buys one to get 45 or 50MPG. It would be nice. Especially with it using premium gas. Premium gas and a small 14 gallon tank doesn't add up to long periods of time in between fill-ups.