1 extra stack of GL when in Fists of Wind means you do more damage in Fists of Wind than in Fists of Fire.

Basically, once you're 76 or above you always fight in Fists of Wind and maintain 4 stacks of Greased Lightning (apart from the very start of your opener where you start in Fists of Fire and swap to Wind after 3 stacks, but for the rest of combat you'll be in Wind).