We’re talking around 22-23% haste and then put a bloodlust on top of it.
A friend of mine Pogthax, who is part of the mage theorycrafting does it, but it’s also kind of situational even then.
He gets combust back so quick due to his gear that he won’t get 3 pf back, so he plays his combust around only using 2 pf.
Even with my measly 14% haste I would have to use shifting power in combust if only using 2 pf, if i bloodlusted.
edit: i am also using a haste trinket which definitely scews it quite a lot, bringing me pretty close to his haste for that duration.