I wrestled and boxed growing up, I was very good at the former and quite average at the latter. I would always reason losses in my own head by saying "well I lost beacuase the guy just has better hands, if this were a fight I could just put him on his back and pound his face in" or "well he can beat me in a wrestling match but anyone at the level of wrestling to beat me can't beat me in a stand up fight and I'd see his takedown coming a mile away then".
After graduating High School I started training BJJ and MMA. I found that i was very well suited for it, I was in great shape and still had my High School wrestling work ethic. So I decided to take a fight. The training went fine, I was injury free and felt extremely confident. But a couple of days out my mind started to panic, what if all this training doesn't pay off? What if I get embarrassed?... It was 10X more stressful the day of the fight but I managed to win fast enough that the adrenaline was still coursing through my veins. I had 1 more fight that went the same way , then I had a fight go the distance and found out how badly my body and mind reacts to the dreaded "adrenaline dump".
I found that if I smoked just before a fight I was much more calm and would perform much better (I was happy to give up some speed for the ability to get my mind right). The last fight I had was a few years ago in Cedar Rapids Iowa at the ISCF MMA Amateur championships. The only problem was I wasn't able to "relax" the way I used to since I was in a town I didn't know. The semifinals went fine, I won (by anaconda choke after crushing the guys ribs with a kick) fast enough that the "dump" didn't have a chance to rear its ugly head. The finals went terrible though, I went out and tried for the same choke and that mother fucker wouldn't tap and when he didn't, my body told my mind "fuck you pussy, you're on your own". 5 minutes later the ref was pulling him off of me and I barely had the energy to stand.
I have since "retired" but looking back at that fight I know If I could have just gotten my head right I would have EASILY won. I'm not assuming Nick Diaz has the same mental block as I do (he probably doesn't) but thats one mans argument for why pot can be used to enhance someones performance.