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There's more to lean burn than just the ECU. The actual engine is able to get the most out of lean mixtures. I know that the HX is effectively a SOHC motor at low RPMs, and opens up the other four at higher RPMs. As for the VX, I have no idea. Not to say that you can't lean out any engine, you can, but you won't get the same results. I've got a mild lean-burn mod on my Cavalier, and it seems to give maximum benefit at around 16:1 AFR. Much leaner than that, and economy actually drops. Parts of the fuel mixture actually get too lean to burn.
Just like the VX/HX, I needed a wideband sensor to do this. You'll also have to install one to get a reliable lean burn. At this point, you can go for the VX/HX ECU (if all the sensors are compatible), or you can fashion a circuit that will train the stock ECU around an AFR of your liking. It's easier to pass state inspection this way. Just crank her back to 14.7:1 and she passes with flying colors.
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