This "may Sayer" has been around Fox Body Mustangs for over 20 years. I've yet to see this 30 mpg out of a heavily modded 2.3, period. I don't know if this link will work below, but the turbo 2.3 was only rated at 2 more mpgs. In my experience, modding a 2.3 usually hurts mpgs, and typically stays the same for a 5.0 version. You can't hardly find those turbo 2.3's around here, and when you do, they are always more than a 5.0 is.
http://www.fueleconomy.gov/feg/Find.do?action=sbs&id=1821&id=5380
Edit: Another point I'd bring up is all of the 2.3's require higher octane fuel. I still run 85 octane in mine.