Hmm, if you have time, one interesting thing to try might be putting together a model of a paved surface and then doing AO + heightmap renders to see how the two compare. Just to get an idea of how accurate it is to be using the normalmap's alpha as both (wear takes off the top of the heightmap first presumably)
The issue I see on switching from envmap to ambient light is, ambient light doesn't have a direction to use in fresnel calculations - so you have to pick some model for the ambient light in the direction it should be coming from, or it'll look like there's sky everywhere. On upward facing surfaces like the road this is not a problem, any viewing angle will be seeing their sky reflection. Not sure it'll work as well on other props.
The issue I see on switching from envmap to ambient light is, ambient light doesn't have a direction to use in fresnel calculations - so you have to pick some model for the ambient light in the direction it should be coming from, or it'll look like there's sky everywhere. On upward facing surfaces like the road this is not a problem, any viewing angle will be seeing their sky reflection. Not sure it'll work as well on other props.