Are you getting the bouncing
Are you getting the bouncing idle problem?
I don't really think you'll be seeing much difference in MPG. Like the idle screw, such a small amount of air passes through it that once the throttle plate cracks open, it's effect is reduced to almost nothing. You will idle a little smoother, but since MPG while stationary is already zero, that's not an improvement!
I had a bad IACV a while ago. Just took it off and sprayed the crap out of it with throttle body cleaner. Seems to be working fine now, nice and steady idle.
Scouring the country for an excellent condition Civic VX