Depends largely on how you drive with it. It's VERY tempting to drive harder than without the kit. I'd say that the best I've got is on the freeway at 70mph where I can get 35 mpg (US gallons which are smaller than imperial). However it's jsut as easy to get around 23mpg around town driving "vigorously".
See GBMINIs MPG & mileage chart; the thin jagged line is miles between tanks (the few very low ones are "short fills" before a trip or similar); the MPG is on US gallons!
I got the JCW-200 in Jan'04 and upgraded to the -210 in Oct'04. So only two fills since the upgrade, but both over 300 miles.
The big driver of my mpg/miles-per-tank is if all my trips are just to and from work (only 11 miles each way) or if I do longer runs.
NOTE: I don't drive *hard* ... but I do have fun