I leave my bars on and have 3 racks…I didn’t notice a real difference when they were first fitted (actually I didn’t notice any real difference until about 8 months ago), I’m now seeing a 10mpg reduction (sadly keep track of mileage and fuel stops), but I think this is to do with other things – like perhaps a slightly heavier foot or not changing up as quickly as before. I think the bars and racks might have a 2mpg impact; with bikes on, it is much more, but having more than 1 frequently isn’t common; 2 now and again and 3 rarely – so the mpg drop isn’t quite as noticeable.
Don’t have space, time or interest in removing them and reinstalling them…so they stay on.
Strangely, I think they make the Fiesta look better than with a plain roof (although I may be falling victim to the ‘lifestyle choices’ mince there!).