I haven't replaced the clutch on my 919, but replaced them on my old 1980 CB900. I'm telling you, I bought regular springs and only replaced 3 out of the 6 and that clutch was stiff as shit for a more than 1k miles. I don't know what the modern CB919 s like, but if you're looking for ultra heavy duty springs, you might want to start exercising your forearms now.
I'm interested in how this goes though as my 919 has 26k on what is more than likely the original clutch and I'll be looking to replace mine in another season or two.
Slow is smooth, smooth is fast.