The engine runs rich on regular to fight detonation at such a high compression ratio, evidenced by the soot build-up at the tail pipe. If you have that, pull a dipstick and smell the oil. It will have strong gas odor.
I have a little soot at the tailpipe of my 2.0L CX-5 but my take on that is it's due to the rich warm-up coupled with the strategy of early exhaust valve opening to heat up the catalyst sooner during cold starts. I don't have a strong gas odor on my dipstick.
Premium fuel does not detonate as readily, so less fueling required to combat detonation. That's how the theory goes.
I saw that theory presented on BITOG also. I'm not convinced because the theory assumes the engine management regularly detects detonation and richens the mixture in response. If that were the case, running premium would give a little spunkier performance due to the leaner mixture and more advanced ignition timing but that was not my experience the one full tank of premium I've run. Also, I know timing is dialed back to avoid detonation (typically not in response to detected detonation but to help prevent it from occurring in the first place). Has anyone presented direct evidence that the mixture is simultaneously richened? Because that could be problematic from an emissions standpoint. In addition to the above preventive strategies (to avoid detonation) it has a knock sensor that can signal the engine to take more drastic measures should detonation actually occur. If the detonation sensor was coming into play on a regular basis, then premium really would give stronger performance.
That soot at the tailpipe further raises the question about what ends up at the intake valves through EGR and PCV. But no engines had been destroyed because of the valve deposits. Yet?
That's a good point. I think the fact that, after more than 4 years, there have been no problems reported speaks louder to the effectiveness of whatever anti-soot deposit strategies Mazda implemented than any speculation we can muster. Still, it's fun to speculate.
I bet the catalyst warm-up strategy of rich running coupled with early exhaust valve opening has a cleansing effect on the exhaust valves. Before the valves are hot enough to carbonize, they get a flow of unburnt fuel over them.