you wont get much better fuel economy.
while off boost and in closed loop mode, your A:F ratio's are probably close as possible to 14:1.
when you get on the gas, then it gets richer and richer.
so in standard form it might get to about 10:1.
an SAFC or any air flow meter signal bender for that matter, will only make an adjustment at full throttle type position.
so you can adjust that 10:1 to about 12:1.
pick up a hand full of KW's while your at it and the the car will use less fuel (JUST) when it is on full throttle.
I doubt you will actually save any money though, considering the car will have more power and be more fun to drive, you will be up it more often.
I had an SAFC on my S14.
it gained about 11rwkw on the same boost just by getting the mixtures to 12:1.
fuel consumption was still the same.