Has anyone thought about putting a set of 2018 STI wheels on their Ascent? I was thinking of doing it but wanted to make sure that the wheels will fit properly.

Yes, they fit.

Are the STi wheels rated to 1800+ lbs GVWR per wheel as required for the Ascent? Subaru could not tell me.

I'd thought about using my stock 2017 WRX rims for 18" snow tires on the Ascent, but instead found a barely used set of Ascent 18" alloy rims for $580, while I'm using the 20" Ascent rims for the OEM all seasons.