How many miles on the clock? Original O2's with high mileage (over 100,000) means they are most likely off with their readings anyway.
My take on your observations is this:
With a MAF based tune, the ECU uses MAF to estimate fuel required, then checks this against the O2 sensors as a comparison for fine tuning the trim and base marking the MAF readings. This requires that both sensors are in good condition or at least within spec.
As the O2's go bad, they can give poor O2 readings and throw the mix out which may cause increased fuel consumption at that point.
Once the ECU faults the sensor, it stops using its reading in fuel mix calculations, and might very well reset the MAF values to factory. That would explain why your fuel economy has improved.
If I'm right, once you replace the sensors with new ones, the improved fuel economy should remain, although this will be dependant on how good the MAF is, and whether it leaned out the fuel trim once the O2 sensor was faulted.
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Cheers
Marty
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Current:
2001 Audi S8 - Brilliant Black with Black interior, C5 RS6 rims (whenever I actually put them on...), Solar Sunroof, Tinted side and rear glass, RNS-D, Grom, Bose, clunky old phone in arm rest!
2002 Audi S8 - Project
Replacement head coming arrived  thanks to MJ
Silver with Black interior. All features as the '01, with the 'S' mode auto shifter. Dodgey rear tint (need to find a way to get rid of that).
Family:
2009 Volvo XC90 V8 R Design - has a louder more obnoxious exhaust than the S8, sounds great! Love this thing - Q7 was double the price, and certainly not double the car!
Sold:
1997 Audi A4 - Hamilton's Club Sport, Achat Grey (will miss the old girl)
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