A lot of it depends on the base material.  I know of a 0.6um or 0.8um technology with 1 lvl metal that was able to differential differences in 10's of uA to 100uA.  With the tighter pitches and more metals, there is a lot more chances for reflection of the emissions on the metal layers.  With three or four levels I don't expect to find anything until I have at least 1mA, and really four levels makes things iffy.  Anything higher, and most designers that I know don't ever expect to find anything with top-side EMMI.  That's when bottom-side starts coming into play, but it is a whole lot harder to setup.