More debate than answers, but this is a thread I found back when I was messing with mine:

DIY-disabling EGR and Air recirculation!

I don't have a California car, yet I do have an ARV, so that's one myth busted right off the bat. With EGR disabled I noticed really bad mileage until I also disabled the ARV (didn't know the two were related at the time). I'm still curious as to why though.

-Rog