Personally I think any combination of slayer and leech is overpowered.
Since leech is a % damage, the moment slayer comes in thats 2-3x damage which is also factored into the leech.
A Mana leech weapon gives players the ability to get more specials in. Hence the AI/crushing blow chains. (or LS though at 3 mana cost that really isnt too much of a consideration)
Life leech is free healing so players are staying alive longer while they are hitting.
Stam leech maintains the swing speed so that their dps doesn't fall off during the fight.
Hell I could give out an undead slayer with 100% energy damage and 10% mana leech and a player could fight shadow knights in doom and keep concecrate on indefinitely without having any mana regen.
I hardly think its a coincidence that the easiest boss mobs in game are those vulnerable to imbuable slayer types.