VERY interesting thread! Never changed a head gasket on a car or moto, how skilled of a mech do you have to be?
you really dont have to be super skilled, but you do need to be methodical and know what you are doing.
taking the head off would require removing all intake (throttle boddies, airbox etc) as well as header, draining of coolant and probably removal of radiator.
Its no small job on the 919, or any motorcycle with overhead cams, especially for the small possibility of minimal horsepower increases.
Typically one will run a thinner head gasket / deck the head and or cylinders, in combination with other mods (hotter cams, cam timing, ignition timing and fuel mapping) to get moderate increases in horsepower... but just a thinner head gasket in general wont do to much, especially on the 919 which its mild cams and small throttle bodies.
Really and honestly the 919 tops out as far as speed wise at speeds WAY WAY to fast for street use... if you want the feeling of extra power, just drop one tooth in the front... walla instant power that doesnt require messing up a perfectly good motor.
12hp gains are nothing to go crazy over IMO... thats only ~ 10% increase, at the sacrafice of actual street power (low to mid range), now if it was making 10% more across the board then ya go for it... but its not especially with retarded cams.