Well it isn't all that simple. It is possible to increase NOX (smog component) emissions and improve MPG by leaning out the mixture. But there are MPG improving techniques that reduce all emissions too. I would think a cam that did more of an atkinson cycle would improve emissions and efficiency.
But NOX does peak on the lean side of stoic, and cars designed to lean burn have special catalytic converters (and probably still aren't sold in CA).
