An ironman friend tells me his VO2 max is 6.0 L/min
He weighs 80 kgs and has a resting heart rate of 40/min with a peak
heart rate of 200/min.

At rest, his VO2 is under 300 ml/min, based on Brody's equation as
used here:

http://www.ncbi.nlm.nih.gov/pubmed/7623055

Lets say my friend can double his left ventricular ejection fraction
during exercise. Since he can increase his heart rate 5 fold, he is
then capable of increasing his cardiac output 10 fold.
Since the capacity for O2 transport by the blood is directly
proportional to cardiac output, that should bring his VO2 from 0.3 L
at rest to 3L per minute at VO2 max. How can I explain his measured 6
L VO2 max? What am I missing?

Regards,
Hansel de Sousa
Nazareth, PA, USA