From personal experience, you will feel a bit lethargic at that high of a hematocrit when just sitting around. However, when you do excercise, your capacity to carry oxygen is through the roof. I was a triathlete and I dont remember my wattage now, its been 15 years but the increase was substantial. It took my average speed on a 40k time trial course from 24mph to 27mph along the same course. I went from placing in my age group to placing in the overall and giving the pro's a run for their money on the cycling leg. In group rides where i had been evenly matched against the faster guys, i was easily riding off the front at will, when ever i wanted. In endurance sports, there is no equal to a high hematocrit. EPO is a hell of a drug.
You have to be careful though, as others have said. To high and you can die. I had bought a centrifuge and measured my hematocrit daily when i was on.