The Insight has been known to run up to 25:1. No amount of lean-ness is too lean.
Its just the leaner you get the harder it is to get complete combustion because the fuel molecules are farther apart and the whole thing burns cooler.
When you say ONLY the head, intake, and possibly pistons changed, those are very major changes. Without them you will not be able to run anywhere as lean as the lean burn engine. Lean burn engines are designed to be able to cope with that cooler combustion by swirling the mixture up better so there aren't any lean/rich pockets in the chamber. This allows running the engine leaner than normal. I think it is around 16.5 or 17:1 is where you get peak NOx emissions, so you at least want to go beyond that so that NOx starts to come back down again. This is the stuff that causes acid rain, smog which causes lung cancer, and asthma as well as probably more junk that is pretty nasty.