In the garage's defence, it does sound like a slipping clutch although it would be very unlucky to have that problem at such a low mileage.
Also never heard of the 'dropping a cylinder' issue you mention - pretty serious in a twin-cylinder bike!
I'm not expecting great things from my new NC750x when I pick it up. I have a four mile commute. Currently get 41mpg from a CBF500. I once got 24mpg out of a Blackbird on the same route