When considering if a pedal can run at 18v...
1. Does the pedal have a chip that increases the voltage internally (like a klon)? If so then it's usually a bad idea to feed it 18v. The angry Andy + doesn't, so we're good there.
2. Did you use components that rated for more than 18v? The ones you typically need to watch are the electrolytic capacitors, which can come at 16v.
The one thing in not 100% sure of on that circuit (but I think it would be fine) is the Zener diode on the boost circuit (build docs show a 12v Zener). More than 12v wouldn't break the diode, it would just conduct through. So if there was more than 12v at that point in the circuit it would cause clipping. I don't think that would happen, and I don't see any harm in trying it.
