Update.
Just got back from the dealer.
They removed the rubber pad over the charger, cut off the ribs from the bottom. (They said there was too much of a gap), then reinstalled the pad. Phone charged from 86% to 100% in 20 minutes.
Go figure!
I know this reply is a bit late, but you can probably still improve your charging, if you want to try this.
First, backing up to explain why the grooves are there that your dealer cut off. GM vent AC air between the rubber phone mat and the wireless charging coil underneath to keep the charging unit and the phone from over-heating. When your dealer shaved those grooves off, they moved the phone closer to the coils which helps with charging but cut off the AC flow.
If you live in a cool climate or don't have much direct sunlight to where your phone sits, this may be irrelevant. If you do however live in a hot/sunny climate, if you pull your rubber charging mat out you can see where your vehicle has slot the AC air comes out. If you are willing, if you just notch that spot in the mat out with an razor blade the AC is more than sufficient to cool your phone off and let it charge at max speed more of the time.
You can even see this if you just pull out the rubber mat entirely and just lay your phone onto the spot where it should charge and leave it there. My phone + charger is 3x cooler to the touch w/o the mat there and charges faster than it ever did with it in place.