The drawback, when blood gets this thick (hemtocrit over 50%), if you get dehydrated, the blood can turn to jelly and BAM instant heart attack.
voodoo alert voodoo alert
Although Hct's above .50 are rare although the normal range for adult males is from the mid 40's to the lower 50's. If the TDF riders are limited to a maximun HCT. of .50 I can tell you they arent going to dehydrate themselves to the point of sludging thier blood. I'd have to do some math but you'd have to loose alot of circulatory volume.