The amount of life taken is multiplied by the above values and converted to healing on Anub. The healing is then reduced by 50% if MS is on Anub.
Here's part of the log report from our normal mode kill last week:
[21:47:52.152] Gaerthe afflicted by Leeching Swarm from Anub'arak
[21:47:53.778] Anub'arak Leeching Swarm Gaerthe 2265 (R: 252)
[21:47:54.508] Gaerthe Leeching Swarm Anub'arak +1756
[21:47:54.969] Anub'arak Leeching Swarm Gaerthe 2061 (R: 229)
[21:47:55.742] Gaerthe Leeching Swarm Anub'arak +1597
[21:48:24.851] Anub'arak Leeching Swarm Gaerthe 382 (A: 779, R: 129)
[21:48:25.521] Gaerthe Leeching Swarm Anub'arak +900
[21:48:26.035] Anub'arak Leeching Swarm Gaerthe 805 (A: 671, R: 164)
[21:48:26.706] Gaerthe Leeching Swarm Anub'arak +1144
As you can see, Anub puts up the debuff, then starts leeching life from me. He heals for 77.5% (1.55 reduced by 50% from MS) of the amount of life taken. Interestingly, the amount healed is calculated after resists but before absorbs. Bubbling people has no direct effect on the amount of healing he receives, although it has an indirect one in that people can sit with lower HP without fear of dying, thereby healing him for less.