It sounds to me like it is your torque converter locking that is giving the increased MPG.
To test this you might try tracing your TCC circuit and then wiring in an over-ride switch.
Most newer vehicles use a PWM type of TCC activation strategy to make this function seem smoother to the driver. Even my S10 had this.
An over-ride will force a signal for 100% lock-up of the TCC at will, and will instantly show you the gain on your S.G.2.
Some (perhaps most) TCC circuits supply 12 volts to the pressure solenoid, and pulse the ground signal based on calculated duty cycle. In this case you would over-ride the ground signal using a SPDT relay.
Single Pole Double Throw (SPDT), Single Pole Single Throw (SPST) Automotive Relays