Well, I think the most extreme case I know of is the gen 3 prius. It runs up to a 30% EGR mix. I'd imagine on that engine not having EGR would be a huge detriment. On any other vehicle who knows. It all depends on the design of the engine.
I
started a thread a while back about using more EGR to create a lean burn like effect. I haven't ever had the chance to test it though.