Athletes elsewhere train at high altitudes to acclimatise to lower oxygen tension in the air. Training with lower haemoglobin will help stimulate your marrow to produce MOAR red cells.
If it was a way to boost performance, you'd have a queue... I think it's different removing red cells, so that the body needs to rebuild them, from training at altitude, so that the body needs to adapt and produce extra. The latter will give you a boost in haemoglobin that will last for a few weeks and enhance aerobic performance, the former will simply restore you to your normal numbers over time.
I suppose this time of the year I could give blood, but for 9 months I am involved in some form of racing or training for said races and I wouldn't want to throw away a lot of work.
Besides, I am AB+, not exactly the most in demand blood type