I am running 34mm flat slides on my 840 GS with an GS850 head (34mm intakes at the opening nearest to the carb intake)
Some people think that the 34s might be to big but its up on the dyno as we speak
I would think that if your going to run a 910 motor or larger, you could run the 33s
The flat slides seem to run better than the old round slides but if that's all you have then that's all you have
good luck
Jim