Anyone know by any chance the outer diameter of the OEM flywheel (I presume aftermarked are same) - I'd like to estimate gains in torque by using a lighter flywheel, and for that need weight and diameters...

 

Technically you won't 'gain' any torque. The only true gains you'll achieve are based on efficiency losses experienced by accelerating a heavier mass (frictional losses). In this example - that adds up to a very small gain that's not worth measuring.
Torque isn't 'lost' with a heavier flywheel - it's stored as rotational kinetic energy.
The increased resistance to changes in rotational velocity (positive and negative) experienced with a heavier rotational mass is a barrier to responsiveness. A body in motion tends to stay in motion - this translates into a slower revving engine. It takes more energy to spin the heavier rotational mass - and to slow it down.

The most important information you're missing involves the weight distribution of the mass along the respective flywheels' radii. This distribution of mass determines the rotational inertia of the individual flywheels (stock vs. aftermarket).

For example: Let

 

slipstream444

Thanks for injecting some factual information into the discussion

 

Thanks LD. I just want to contribute and inform when I can - I think that's what this forum is all about my friend.
I've learned a LOT from major contributors like Road Rage, XViper, Billman250, Slows2K, CoralDoc, yourself ... and many more. Contributing info is a small way of paying you folks back.

Cheers.