I had the chance to do this recently and I immediately noticed a change in the mileage of the car..... I had a MAP ENHANCER installed (potentiometer from MAP sensor output to ecu) and adjusted it while looking at the VF pin output until the numbers started changing from 2.5V DC....
From that time on, it was running much better and more fuel efficient.
That was until spark plug 1 went out and I thought I broke something.... I even reset the ecu thinking it may change something and never did until I checked the spark plug..... only number 1 had black dry deposits..... all other ones had the usual tan brown on them.
changed it then regretted messing around with the potentiometers before.... I will need to check again to see if the map setting is dialled down again.