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any pressure build up (aside from normal compression) that occurs prior to tdc is counteractive, so, under certain conditions, you can get some improvement by increasing the rate of combustion. Also it is generally accepted that a pressure peak of about 10 degrees ATDC is most efficient.
Since lean burn is "slower", as is lower pressure, the pressure curve is wider (or as the curve is wider in relation to higher rpms), adding n02 to enhance combustion makes sense so that you can narrow the pressure curve and put more on the expansion side of tdc. Again, the effect depends on conditions.
As well as "leaning out" an engine is somewhat counter productive, the engine is oversized and moving more air than is needed for combustion. Lean AND high rpm is a bad combination.
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