Yes increased mitochondria density is a great subject by itself.
Cardio vascular performance I was under the impression is based on
I) how many beats your heart does
II) the amount of blood it pumps per beat
III) the power of the heart can contract
This means more blood being pumped in times of training and thus more oxygen being pumped around the body. However, during the endurance cycle it's fuel + oxygen to create ATP via the mitochondria and more of them would help!
Just to confirm VO2 max is the maximum oxygen consumption. So.. in theory we are talking the same thing. AS more mitro is more oxygen and a stronger cardio vascular performance is more oxygen
so E.G HIIT increases mitro and not cardio performance (https://www.ncbi.nlm.nih.gov/pubmed/20473222
SO... my theory is that medium heart rates of 65-80% would mean your body adjusts to the situation and creates higher cardio performance... which is the point of zone 2 training... which could lead to more optimised fat burning and a "fitter" heart.
I'll go research