Carbon build up is common and will cause pinging. So will "hot spots". A properly running engine designed to burn 87 octane fuel will in fact make more torque burning 87 octane vs. 93 octane. Higher octane fuel burns slower and colder while requiring hotter ignition to combust. A lower octane fuel will burn quicker and hotter and not need as much heat to start the combustion process. This is why higher octane fuels resist detonation (or pinging). The reason you experience more power with the higher octane is because it is resisting detonation and firing at the right time (expanding and pushing the piston down). When you run the lower grade fuels, the air/fuel is igniting too quickly and the impact of the force applied on the piston is at or a hair past TDC. Think of it as a bicycle. When you push on the pedal with your foot, you push as the pedal goes down. If you push when the pedal is at its highest point or just after, its extremely hard and not very productive. This is what is happening in your engine.
Before someone says "if higher octane fuel burns colder and slower than why would race cars use it". This is because of the extreme cylinder pressure. Pressure creates heat, and when used with lower octane fuel, that heat would pre-ignite the fuel and you have detonation and soon a trashed engine. Likewise, you can advance the igntion when running higher octane fuel because it burns slower.
