It's been my understanding that a key to developing aerobic power is to increase hematocrit by causing new red blood cells to be created. This is what exogenous EPO does. However, it's also my understanding that this is the purpose of Z5 intervals: to create sufficient oxygen debt for the kidneys to kick out endogenous EPO. Hence the emphasis on doing that Z5 work. IIRC one has to get oxygenation down to 87% for this to happen, usually done with 3X3' intervals. I have an oxymeter which I've used for altitude training, but haven't experimented with it on the bike.

I haven't read any research suggesting that high volume z2 work would have the same effect.