To maximize the MPG, you want to get the trans to the highest gear as soon as possible. The thermodynamics are complicated, but generally the engine is most efficient at burning fuel when the combustion pressure is the highest for a given output, and this happens when the engine speed is as low as it can be (without lugging) for a given steady speed. So for the shift table you would want to fill the bottom right part of the shift table as much as possible with the highest gear.
I had a Passat that would cruise on the highway at 1450 rpm at times, which delivered great mileage (for a vehicle that size). My old Corvette can lope along at 1600 rm on the highway (though with 407cid = 6.7 liters and 450 hp, the mileage still isn't great!). The overdrive transmissions have made a big contribution to this reduction of RPM, of course. But these low rpms are only possible if the fuel can be metered accurately at low rpms and moderate loads, something that carbs weren't very good at. That's why you saw the mass introduction of overdrive transmissions AND EFI almost simultaneously in the 1980s.
For preventing downshifting, you have the shift table plus the deceleration mode settings ( http://www.msgpio.com/manuals/mshift/V5tune.html#gh ). For these, in your case you probably want to set:
- the TPS threshold high (so it prevents downshifts min more circumstances),
- the max. load low (so a higher target gear is chosen), and
- the delay longer (so prevent it from timing out unnecessarily)
Then you could start looking at secondary effects, like reducing the line pressure to reduce hydraulic pumping losses, and reducing PWM% for solenoids to reduce electrical drive requirements (though these effect fuel efficiency much less than getting the shifting right, and in some cases they contradict each other).