Yo you can get the same gas mileage as an NA car trust me. Its all about the way you drive and your tune. Comparing both cars such as an Evo and an S2000. They say an Evo gets 18-23 mpg and the S2k gets 20-25mpg. Thats if you take both cars to 3000-3500 rpm in each gear. Sure once boost hits, you are better off with an NA car, but thats the advantage with boost if you are a good driver. Boosted cars usually have a low compression motor. In general a low compression motor will get better gas mileage than a high compression motor. Now an Evo comes with a 4G63 motor with a compression of 8.5:1 and the S2k has like an F20 motor? with like a 11.2:1 compression, one of the highest compression street cars on the market.

I usually shift around 1800-2400 rpm on economy driving and I usually take my motor upto 6200-6500 rpm on a 7K redline while pulling and usually I shift from 3rd to 5th gear and I get a gas mileage of 20-22 mpg. My personal best is 28 mpg. Since a turbo is run off your exhaust gases, your aren't going to loose that much gas while boosting upto 3k and shifting in economy mode. I mean use reason and shift at low rpms, when you are downhill or slowing down to a stop light, just throw it into neutral.