I still play competitive rugby so my cardio is pretty good but I definitely feel better if I can dive higher % of 02. Just my experience here and no hard science to back it up other than my own feedback from my body.
Well i also play the same (as well as getting into running lately) and for backgas i notice no difference at all air to nitrox in terms of fatigue and so on. Im still dog tired after a day diving no matter what i do (usually due to 2-3hrs sleep and so on).
However i DO notice a difference doing deco with 100% vs 80% and so on in that i get out feeling better.