[Finnegan]

What are your objectives? Fuel economy, fun, or something in the middle?

If you're after better mileage a Z4M with a blower isn't going to get you there. You'll probably be in the same range as the M3 if you're getting 11 MPG with the S65 -- I got around 16-17 mixed driving with my E92 M3, so you're driving style + blower, which is insane fun, well, you get the idea. Again, blower on a Z4M--so much fun. Put some coils on it, a few other things, it's a wonderful hard edged and fast machine.

The N54 (E89) gets good mileage cruising, better than the M, but it can drink gas too based on driving style (and tuning). It's also a very different car than the M, softer, heavier, not as raw. More of a cruiser than a hard edged sports car.

That's my two cents. Tell us more about goals/objectives--that will help.