Your Aerodynamic factor will only produce negligable Fuel mileage drop...
If you really want to taste what alloys look like and you are ready to fit them in your car,i feel its majorly a preference of looks over a minor issue like fuel economy...[it wont drop that low with 15 inch rims,its the tyre width which plays a role in fuel economy]
Probably since you have got your side body spoiler kit,GC is more of a concern than the ones you mentioned above...
15 inch alloys with 195/60/R15 Tyres is the ideal upsize for you...Go for it!
As far as alloys are concerned,Check few of them here,Hope you like some...[Image source:GOOGLE]
Success is not final, failure is not fatal: it is the courage to continue that counts.
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