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In TDIs, lots of modding folks disable their EGR for getter performance or prevent intake clogging from the mixing of soot from the EGR and intake oil coming from the crankcase vent and turbo seal.
Disabling the EGR have been shown to reduce FE by as much as 10% on TDIs. So it would make sense that increasing EGR could surely increase FE up to a point, but reducing power. The EGR value in the TDIs ECU can be reprogrammed up to a certain point without throwing a code, but I've never heard of anyone tweaking the value UP for better FE. I might give it a try someday but my vag-com cable (kind of an OBDII cable for VWs) is toast.
With diesels the air charge is more or less always maxed out (there's no throttle plate and on TDIs there's a turbo pushing as much as 15 psi), so they pretty much always run lean, we only play with the amount of fuel injected for the desired power output. It is pretty evident in diesels that more EGR means less fresh air charge hence less injected fuel but also less power. So it's merely a question of FE/power balance.
I don't know much yet about gasoline engines so I can't say what increasing EGR would do.
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