I'd take and M3 over a Mustang GT any day of the weak. The quieter and smoother ride that would destroy the GT on my streets because of the curves in my area. We're lucky to get a 100yrd straight away. Down a 1/4mi strectch of our rode a stock RSX-S would destroy a stock 72 Chevelle LS6 do to curves but I digress.
I think what the OP is getting at is overall balance of the car and the fuel efficiency should be factored in. With C5 Z06 owner getting 30mpg hwy @ 80mpg avg for a road trip, 405hp, 400lb-ft and still hang with a 911turbo/360 Modena in the curves for a fraction of the cost is incredible. Chevy got it right with this car (well dash materials could be slightly better). But how much of a freak'n snob would have to be to let that deter you? The C6 Z06 has been known to get 27mpg around 82mph and the braking, 1/4mile, skidpad and emergency lane change ability is insane along with it's near 200mph top end.
If I had the money, I'd take the Z06 over any other car. If I was only allowed to get a convertable with $$$ as no limit...360 Modena-just look at how sexy she is and since you can't get the C6 Z06 in a convert!