:hey Martin I didn't know you were in touch with the dark side lol
If you haven't solved this yet one........ thing test comes to mind that might be worth doing.
If you ride at 50mph steady speed with a wide open throttle and then back the throttle off ever so slightly like about 1/8th throttle and note the change in engine note and performance .
if the engine hesitates slightly then pulls strongly it indicates that your WOT mixture is too lean and you need to increase the size of your main jet.
if the engine just bogs and there is no sign of any improvement then that would suggest a over rich mixture .....which would be borne out by your spark plug .
Let us know what the result is