Have had the Crosstrek for a year now and the average over 54 tanks is 8.9 L/100km. Not that great, especially since 90% of our driving is highway at 100-110 km/h. Best ever tank was 7.3 last summer. Worst was an 11.2 just recently in the cold snap.
The roof bars and ski box have been on the car pretty much the whole time, though. Took them off for a month in the summer and the average did drop down to 8.0 for those seven tanks.
I'd like to take the bars/box off in May and not put them back on again until November, but my wife likes having them for stuff in the summer (hiking gear and whatnot when she's got a dog and three friends in the car.) The car is kind of small, so I agree it's often handy to have the box, even in summer. I suppose I could take it off and put it back on only when we need it, but I'm too lazy.