Since I was able to finally log on to the EU realms again, I could do some testing on the Blood Parasite on the heroic testing dummy in Acherus.
What I can say definetly are these things: Bloodworms healing and Health is based on your own health pool.
with
33850 Health they had 8164 HP and healed me for 408 per stack of Blood Gorged.
58400 Health they had 14192 HP and healed me for 709 per stack of Blood Gorged.
They always spawn inside the Mob and then move behind it, and do not seem to have any sort of icd or ppm, since I had everything from 1-4 at a time.
their damage seems fairly random and ranges from 200-280 per hit when fully equipped.
Their heal can crit, but I only had the heal crit once in 20 worms.
Now, what I'm not sure about are the stacks of Blood Gorged. It seems to be moderately intelligent, since the bloodworms almost always lasted the full 20 seconds (14 stacks) until they burst when I was at full health, but they burst with as few as 3 stacks when I was on ~50%. and 7 stacks when I was above 75%. I couldn't deduce an exact system to it though.
All in all they do seem to be much more useful than the current live version, since they heal for a higher percentage and don't die as fast.
http://www.worldoflogs.com/reports/j84s6dpp49u5aqw0/That's the Stoncecore run I did today. The best boss to look at is Izruk, since he's a straightforward boss, with a bit of AoE damage going around. The worms did around 4% healing with 10-15% overhealing on me. That's not too shabby, especially when considering that the healing gets better the more damage everyone takes. The little overhealing also seems to confirm that the blood burst is intelligent and adjusts itself to the current health.
On a side note: To take Blood parasites I tokk the two points from Crimson Scourge and managed AoE tanking just fine.