I don't run 15psi because that's all the I wanted to run, I run that because that's all my setup can safely run on 91. I just don't believe you are running 18psi with your setup on 91 without detonation. You would be the first. Plus if you are hitting that much boost on the dyno you are spiking much higher on the street. Either your car is a freak or we are saving you a rebuild. This is just a discussion not an attack on you.
Detonation wont kill your engine instantly. It's a slow death and every time it happens your engine becomes less and less resistant to it.