hi fatspanner,
i had exactly the same problem as you are descibing. standard bike,

s and splutters from 8000 rpm upwards every gear. changed jets, changed plugs, slotted pick up, still the same. a real headjammer.
solution: how old is your fuel? it will go off ater about 4 weeks. drain your float bowls and fill with fresh fuel, then run it. yiou will notice a difference. mine pulled straight to the red line no problem.
just a thoght hope this helps.