Great link, thanks! 3% on an Insight is actually a huge improvement because the engine is so tiny already, and has lean burn mode. That said, because of its lean burn design, it probably can handle more EGR without stumbling than the typical turn of the millennium engine.
I did find anecdotes of people tuning big V8 cars for more EGR and getting fuel economy improvements, but not in a controlled experiment. If anyone else finds more articles, please share.
E.g. looking at this target EGR table kindly posted by someone who spent a lot of time hacking their AMV8/Ford EEC-6 ECU:
It's clear there's potential because the OEM added EGR as an afterthought to trim emissions a bit in certification conditions. I have a hard time imagining that EGR would be useless at 4000rpm, or above 50% load, and I have a hard time imagining 0.25% charge dilution is anywhere near the limit in the lower cells.