I don't think the oem 15" wheels and tires weigh much more than the oem 14" wheel and tires. The 14" steel and aluminum wheels were pretty close to the same weight.
I just installed some 15" oem wheels onto a 2023 Mirage GTand they're lighter than the steel wheels and larger 185/60/14 snow tires that I removed(had older 14" wheels on it for winter). I didn't think to run them over a scale at the time.
Unless you have some pretty heavy 15" tires on the 17' GT I don't think you'll notice a much of a difference over the stock 14" tire combo.
View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)