It definitely alters a fighter's performance for better or worse (people react to weed differently). On sidenote, alcohol is banned in many sports that deal with precision, and that's because it lowers heart-rate and increases accuracy.
In this case, weed is definitely a 'psychological enhancement drug', might help a fighter drown out the noise or tension from outside, definitely affecting performance. I'd rather see a fight with two people on PEDs like Japan, an even playing field, than a fight where some performers are taking psychoactive drugs. I do support fighters being able to smoke weed as a form of relief during healing, however.