I made the switch and have not noticed any mileage loss. Tinted windows and the new wheels make my silver 08 civic look like a sports car (I love it). Check it out at
http://www.gestner.net/civic.htm
The new wheels are actually lighter than the Honda originals.
John
PS mileage did start creeping downward right after the wheel switch. After a week of that I checked the tire air pressure and sure enough they were set at 32 lb not the 40- 42 I have them at now. Mileage is back up to 46 MPG average again. I only have 1800 miles on it so far so the hope is for even higher mileage as time goes on.