From FoF #95:
"What, if any, is the effect of Spirit Speak on the amount of mana leeched when in wraith form?"
First, mana leeched in Wraith form will be between 5%-23% of the damage done to the target - with a minimum of 1 mana leeched and a maximum of the target's total mana.
With no Spirit Speak, the mana leeched will be 5% of the damage done (after resists) - so if you do 50 damage to your target while in Wraith form with no Spirit Speak, you'll leech 2.5 mana. At GM spirit speak, you will leech 20% of the damage done - in the above case, you'd leech 10 mana. Legendary gets you to 23%, which gives you a total of 11.5 mana leeched in the above case.
For the math fans, the formula is percent leeched = ((15 * Spirit Speak)/100) + 5