My gas milage has stayed the same, if not gotten better. Rich/lean, I don't know for sure but the IAC does not do much when you in the gas so I don't think it matters. At idle you will be a little richer. Smog is the reason that people who are against or question this plate, do not like it. The main point is "Ford wanted the holes to be that size for a reason, so it much screw something up!" Now the smog testers are mainly concerned with NOX gas which is produced at highway speed, IAC not in use.
The reason the plate on your IAC (an idle component), effects driving between shifts, is due to intake vac. When you let off the gas to shift, your intake wants to create vac. The plate restricts the amount of air coming in and less air helps vac. Same as down shifting, but with out the rpm increase.