Would a radiant wall panel be more effective with drywall tight to the heat transfer plates or with a ~1" air space between plates and drywall? Looking online for examples I have seen it done both ways.

As with windows, an air gap adds insulation. Insulation in a radiator (between source and destination) reduces speed and/or efficiency.

That being said, I could imagine scenarios (like tubing alone, no plates) where air+convection would distribute the heat within the radiator better (ie, provide less insulation) than some materials. Yep, 1" air gap = R1, 1" of soft wood = R1.25.

Plates tight to drywall, thus "radiant'.