From my point of view, it is, in an indirect kind of way. There is no doubt that marijuana provides a pain relieving/relaxing quality to the user. During a fight camp, the fighter goes through a lot of issues, pain being one of them. There are different ways that a fighter can relieve his pain and marijuana could be one of them. To me using marijuana as a pain reliever is an advantage that a fighter may have against fighters who do not use it.
So in this case, could you consider marijuana a performance enhancer if it helps with pain management?