What is the gas mileage in the JCW Sidewalk Convertible?

Should have the information you need at www.fueleconomy.gov.

Not much, honestly. If you can keep your foot out of the pedal, you're not going to notice that much of a difference (1-2 mpg).  Not all MC's or MCS's get the same mpg... some also have more hp (after being dyno'd) than others of the same build...I think we're seing somewhere in the vicinity of 10-20 hp differences on the forums...

Mini.com  check stats.  I believe is around 28 in the city or so

The JCW models are listed at the same gas mileage as the non-JCW cars.  The theory is that, even though there is more power, the car is breathing better because of the intake and exhaust.

Of course, the type of people who buy a JCW car tend to push their cars a bit more than the average person.  Since the pedal ends up on the floor more, real-world mileage tends to be lower.  YMMV (literally)