Wal is certainly right, that if you dive with full packing, your blood is far less important. As an example, when I did the 82m CW WR in 2001 I was anemic with Hb = 13 g/dl. Yasemin Dalkilic has sickle cell anemia and has even lower hemoglobin...
However, when comparing oxygen stored in lungs vs. blood, for diving, remember that O2 stored in the lungs experiences buoyancy change, and therefore has less 'value' than O2 stored in the blood, which doesn't experience buoyancy change.
For example, starting at the 'baseline' level of 5 litres of blood at 14 g/dl hemoglobin, and increasing that to 6 litres of blood at 21 g/dl hemoglobin, you gain an extra
5L @ 14 g/dl = 938ml O2
6L @ 21 g/dl = 1688.4 ml O2
the difference is 1688.4 - 938 = 750.4 ml O2
air is 20-21% O2, but even after ventilation your lungs never hold more than 17% O2 (because of the CO2 in your lungs).
So, an extra 750ml O2, converted into air at 17% O2 gives 4.41L.
So, by improving your blood with the intense 'no aerobic exercise' method, the increased O2 in your blood is equivalent to an increase in lung capacity of 4.41L, WITHOUT any increase in buoyancy nor buoyancy change.
Taking an extra 4.41L of 'real' air in the lungs, would result in an extra 4.41kg of buoyancy at the surface, resulting in far greater effort to get down. Whereas an extra 4.41L of 'air' equivalent, in the blood, results in no greater effort to get down.
Keep in mind that at rest (for example: static), you burn about 250 ml O2 per minute, sometimes as low as 200. So an extra 750ml of O2 is an extra three minutes of static time....or more.
Here is an even more interesting calculation:
1. Diver with full packing
Blood O2 = 938ml O2 (calculated above)
Lung O2 = assume 9L VC with packing, 2L residual = 11L @ 17% O2 = 1870 ml O2
Total O2 store (not including creatine/myglobin) = 2808 ml O2
2. FRC diver with high blood quality
Blood O2 = 1688 ml O2
Lung O2 = assume 4L VC + 2L residual = 6L @ 17% O2 = 1020 ml O2
Total O2 store (not including creatine/myoglobin) = 2708 ml O2
So, the two divers have ALMOST EXACTLY THE SAME oxygen store, except that the FRC diver sinks the whole way down and experiences a higher blood shift, meaning the arms and legs will use less O2 from the core supply on the ascent. Further, the FRC diver can do his max depth over and over and over, one after the other, for training, while the inhale+packing diver can only do one max per day, for DCS reasons.
Also, the FRC diver experiences a dramatically reduced pressure gradient in the end of the dive, since the lungs don't expand nearly as much in the last 15m. This means, from both theory and practice, that there isn't that sudden feeling of hypoxia and fading vision in the end of the dive, compared to inhale +packing dives.
Although I still go to competitions, my main goal is to be able to do repetitive 60-65m 'fun' dives to go and see things and enjoy the ocean (with as short as possible surface interval between dives). So for me the FRC path is the only choice (for DCS reasons). But, each person can choose their own path.
I was ragging Seb about seducing people to the dark side just yesterday.