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.