Changing the '17 BRZ PP wheel finish?
 

If you look at the visualizer on Subaru's Japanese site here, you'll see that the 17 X 7.5 wheel seen on the GT trim looks to be the one we will be getting in the performance package, but it has a gunmetal finish rather than the black finish shown in the US site photos.

How difficult/expensive would it be to have the black wheels redone in that gunmetal finish (at a very high durability level)? I'm wondering if it may be less costly to use the black wheels for winters and buy a new gunmetal set for the summer tires (assuming they are made available somewhere).

Or you could just buy better looking/performing wheels. Preferably something wider than 7.5".

Go to a powdercoater. It'll probably be about $400, and you can get them in any color you want. I had looked into powdercoating my OEM wheels bronze, but never got around to it.