Tested on mobs and dummy on Phoenix: Necromancer level 23 lifetap on lvl 37 mob (Arawnite Shamaness) for 80 damage. Pet healed for 40 damage. On live, Dark SM and Body Cab could heal themselves for more than the lifetap hit for.
Example:
Live Necro casted lifetap: 50%drain
You tap target for 200 damage.
Live: you drain 50% * 1.25 (spec bonus) * 200 = 125
Phoenix: you drain 50% = 100
Live examples:
Necromancer: https://www.youtube.com/watch?v=z8KWNW8un00
50% return
0:21, 206 lifetap, 140 healed = 67%
0:46, 263 lifetap,163 healed = 61%
Dark SM: https://www.youtube.com/watch?v=MgOfMPhRLKI
90% return
0:35, 383 lifetap, 428 healed = 111.7%
https://www.youtube.com/watch?v=68fbN1bLbhA&t=299s
10:21 341 lifetap, 306 healed = 89%
10:22 438 lifetap, 477 healed = 108%
10:30 246 lifetap, 260 healed = 105%
10:31 354 lifetap, 368 healed = 104%
Reaver
https://www.youtube.com/watch?v=k4BWtERZo2I
70% return
1:11, 152 lifetap, 114 return = 75%
Example:
Live Necro casted lifetap: 50%drain
You tap target for 200 damage.
Live: you drain 50% * 1.25 (spec bonus) * 200 = 125
Phoenix: you drain 50% = 100
Live examples:
Necromancer: https://www.youtube.com/watch?v=z8KWNW8un00
50% return
0:21, 206 lifetap, 140 healed = 67%
0:46, 263 lifetap,163 healed = 61%
Dark SM: https://www.youtube.com/watch?v=MgOfMPhRLKI
90% return
0:35, 383 lifetap, 428 healed = 111.7%
https://www.youtube.com/watch?v=68fbN1bLbhA&t=299s
10:21 341 lifetap, 306 healed = 89%
10:22 438 lifetap, 477 healed = 108%
10:30 246 lifetap, 260 healed = 105%
10:31 354 lifetap, 368 healed = 104%
Reaver
https://www.youtube.com/watch?v=k4BWtERZo2I
70% return
1:11, 152 lifetap, 114 return = 75%