Curly's comment still applies.

There is a general marketed "hp number" that a turbo can support, but it's not anywhere near exact, and it's not the same for every car. First of all, the simplest way to bust this is the neglect the engine all together. Weight, driveline efficiency, brake drag torque, tire rolling resistance, aero drag, etc all play into fuel efficiency.

On to the engine. If you simplify everything, as simple as you possibly can, you still have different sized motors. This immediately means the VE is off. Then you start to factor in bearing drag torque, piston ring friction, and all other mechanical frictions. Now think about head flow, intake flow, exhaust flow, exhaust pressure differential, and all of the efficiency factors for literally EVERY part of the engine. Every flow characteristic, friction surface, injector spray pattern, spark burn completeness, has an inefficiency, that will vary engine to engine.

So, all in all, you can create assumptions of "this turbo may achieve better fuel efficiency than this turbo, given the same motor" because you are changing one main factor of the system. You cannot assume anything based off of the same turbo on different car, unless you can calculate the differences in everything I mentioned above, and more (which you can't).

For better mileage I understand I need to lower my cruise kpa (from 60-80).

Should I use a bigger turbo or should I lower my final drive ratio ? or both ?

Current turbo TD04H-13C - Compressor 40/53mm turbine 52mm
for compression GT2560R is Compressor 46.5/60.1mm turbine 53mm
Current dif NB LSD 4.11


Thanks !