Keep in mind that as long as the STFTs are within its operating window then LTFTs values are ignored.
Being +/- 2% is just fine so you did a good job on tweaking IFR and as you mentioned its more constant which is good because that puts less work onto the PCM's slow 17 Mhz CPU and gives it more clock time to do other functions.

Its hard to look at gas mileage on cold winter weather but I'd suspect the MPG will increase a bit in summmer for keep in mind winter gas makeup does not give good MPG.

You might try a test of setting IFR a bit leaner across the board, as long as you do not have ping then engine will perform better and MPG will also increase.

Being you have water injection I would not expect a linear IFR table since when WI is on makes AFR about 8% richer thus IFR needs to be commanded with shorter injector pulse width when WI is on.


Lastly keep in mind if you tuned it correctly for better performance then MPG would take a hit but thats the purpose of performance tune right ?

But you can tune it leaner and still get performance by tuning the timing corretly to prevent the ping yet using a bit more fuel to prevent ping and using higher timing makes the power but in any case your on a better tuning path then you were before and the STFT is saying just that.