Well, Kai once siphoned the entire Magic Purification spell, so I had say they can siphon a lot of magic. Thus, they probably won't be overpowered, though personally I doubt they can siphon them to death. It was theorized that if they siphon an original, they may also siphon from their entire Sireline. If true, this would definitely a magic excess, but it was never shown or confirmed.