You're getting 28 mpg. There's no reason you shouldn't be able to get 40-50 mpg. Time to start re-jetting. Either do it yourself or have a shop do it for you.
My advice is for you to find a good independent shop and go from there. Also do some research (online) and find out what makes your Keihin carb tick. Then you'll be able to talk carb with your mechanic and understand what he's talking about.
This stuff isn't that difficult and you could easily do it yourself with some research and some trial and error.
While they might look "clean" that's not exactly what you're looking for. The color of the plugs is the thing - they should be the color of a weak chocolate milkshake. Not white but a light shade of beige (sorry). When my bike was rich the plugs looked clean but they were very black and the bike ran great. It too got poor mileage.
Much of the time a dealer will simply install a "jet kit" that does nothing more than fatten the jetting way too far (and in the wrong places). My dealer never ever did any plug reading on any bikes I know of. They just put the jet kits in and sent you out the door.