ha thats funny, i was shown this at work the other day. i had a 2k hdr for reflection and a 512k map for indirect bounce, pretty cool. i was lucky the object wasnt moving so i could bake the FG.
i'm surprised that the ao thing works well. i tried it too and got no real noticable results. i guess i was doing something wrong. if you want a dirt map you should asign a new texture bake set and do an occlusion bake. that will give you a good starting point for your dirt map texture. (you need uvs for this tho)