It's a marginal gain, certainly, but VO₂max doesn't rise proportionally to total hemoglobin mass (tHb). Estimate based on several studies:



So if you're one of the lucky ones who sees a 4% gain in hemoglobin from testosterone supplement, you could see a 2.6% gain in VO₂max (in addition to all the negative risks). For most of us, that's about a 1-1.5 point VO₂max increase.

I'd call that amount of gain "modest". And a crummy bargain.

More reading: Saunders et al, Relationship between changes in haemoglobin mass
and maximal oxygen uptake after hypoxic exposure, Br J Sports Med 2013