I think the wraith form mana leech formula may be missing something, maybe it's (SS+1)/5.
The reason I say that is that as a weaver mage with 0 SS, I can leech mana in wraith form too. Elf, so no JOAT. Unlimited FBs and WODs as long as my damage is high enough.
Can anyone know if there's better info on this?
Regarding HP leech, only vamp form gives the inherent HP leech (doesn't leech from damage done by spells). Wraith form gives mana leech (works with spells).
However, whammys can use the curse weapon spell to provide HP leech. It leeches 50% of the damage you deal, more twice the 20% that vamp form gives. Duration however, is tied to SS.