I think we can conclude that aircon on the TwinAir motor does use extra fuel.
Also, I think the more economical your driving style the more apparent it is.
Running the aircon on any car will reduce economy. A/C takes power from the engine, and that power has to be paid for in fuel.
How much, and whether or not you are able to notice the difference, depends on many factors but is basically down to how much cooling is required, and how long it's switched on for.
Ecodrivers will certainly notice a bigger difference, partly because they are using less fuel to cover a given distance, and partly because it takes them a little longer to cover that distance, so it's a 'double-whammy' effect.
In the worst case scenario in my 1.2, running the A/C reduces economy by over 10mpg.