Q: As a percentage, how much are the abilities of blackened Servants enhanced? Or, when such Servants have an excess mana supply like Saber (Artoria) did, can they reach a level of power comparable to or greater than what they had in life?

A: They’re like runaway cars that never run out of fuel. Except, since they’re out of control, fine handling is impossible!

Fate/complete material III; FAQ, pg. 132