I used to get the P0402 code on my 95, and when I did, the gas mileage usually went up, with no ill effects.
There is no way that cleaning a MAF that is already 99.99% clean will make a difference, ie: needing to always have the cleaner with you.
The only time I had an issue with a dirty MAF was when the car pinged at WOT, and the MAF had 150k miles on it w/o being cleaned.
Hopefully some new Motorcraft o2's will clear it right up.
97 T-Bird LX 4.6 - 80k miles
94 Supra TT Auto - street/strip car
04 CVPI- Brenda's car - 76k miles
95 T-Bird LX 4.6 - fully optioned, owned 15 years, 220k miles
96 Cougar XR-7 4.6- Brenda's car, owned 11 years, 187k miles
88 T-Bird 3.8 - first T-Bird, owned 5 years, 206k miles