I think the improved drain soul does not work
it does work, but up to 5 level differences you can gain Soul Siphon
example: if you are lvl60 you can gain Soul Siphon after killing mobs lvl55
(this is just what I have noticed)
I dont know exact amount of mana you can gain from it, just used for leveling
list about broken things is long, its more gamebreaking for PVP