Unlimited Lean Burn?

Hey all, I tried searching to see if this idea had already been floated (I'm sure it has) but I can't find it so here goes:

Has anyone looked into how one could prevent the system from dropping out of lean burn in order to purge? I am curious as to what MPG benefit could be had from removing the cat. and and allowing it to be in lean burn indefinately, whenever possible. (But from what I can find, this might require an ECU mod?)

Thanks,
--Ash