These are just my observations: Unless you bought the one-in-a-million plug that was bad, I attribute the first bad plug to detonation which will definitely give off misfire codes even though technically the engine didn't misfire. Assuming that to be the case, you're looking at bad fueling or bad fuel (which you considered). Since you're also getting misfires on both banks, the thought that all of those plugs/coils going bad at once is highly unlikely. That would return me back to detonation, and all the various bad things that happen there born out by the fact that at low load you didn't see any problems. For the cause, I'd look at the most likely culprit being the fuel or the aftermarket fuel system.