It was a while back that I dug through all this but I think it was this one that pointed to an increase in hematocrit of 4% and RBC count of 9%, and subsequent googling that indicated that an RBC increase of that magnitude could lead to a 5-7% increase in VO2max.

To be clear, I'm not suggesting that this person's increase in fitness is all due to a direct increase in VO2 max from TRT. He trains hard, prioritizes sleep and recovery, and eats reasonably well from conversations we've had. But the first two are significantly improved by TRT (in his own words).