Doesn't that seem a bit much for a CR 8 swarm? Swarms aren't necessarily easy to get rid of.
Exactly. A single yphoz might have a tough time attaching to characters with decent AC, but the swarm is an auto success.
Frankly, I'd rather increase the CR than keep the low Blood Drain.
Going by the numbers roughly 4 to 5 Yphozs in the swarm hitting per round (since an individual Yphoz averages 1.5 damage and the Swarm does 2d6). If each does normal 1 Con blood drain, that's 1d8 Con.
At the very least I think they should do 1d4 Con Blood Drain, like a Dire Weasel or Stirge.
How about they do 1d4 Con normally, but 1d8 Con against helpless (i.e. paralysed) opponents, who'd be unable to brush them off?
While I follow your logic, I'm not fond of the "double whammy" on paralyzed prey. How about 1d6 Con across the board, with a CR increase to compensate?
More or less. But do you agree to increase CR to at least 9?