Efficiency of the burn does not equate to efficiency of the engine–high compression engines suffer terrible pumping loss–the energy to compress is a function of the cube of pressure (I think). So you’ve got this super awesome burn, but you rob energy to compress the air/fuel. Eventually the energy robbing overruns the increased efficiency.
On fuel mixture–High compression should create more potential for power, but you have to enrich to reap the benefit, especially with a high cam profile. The lean mixture combined with the superdense air ends its combustion before the optimal time in the power stroke. A rule of thumb is just prior to EVO.
The burn time in the 12.5+ : 1 compression ratios is getting shorter and starting just as early. A (slightly) greater percentage of the combustion energy is released before the top of the compression stroke.
A high spark advance aggravates this problem further both IRL and in-game–you will notice if you increase the spark advance, then the advance produces great advantage up to a point, then it doesn’t really help or hurt much until it reaches another point where it kills the engine with detonation.
Side note: One missing dynamic is reliability vs. fuel mixture–lean, high compression engines make so much power they put holes in pistons, reforge valves, and crack heads, which is another very good reason to run richer, and all manufacturers do it–adding more fuel to burning fuel cools the cylinder and head.