Does Mamiya sell specific screens to match specific lenses' focal lengths? Are specific screens sold along with each lens? I never heard of this. I've heard of certain focal lengths or apertures working better with certain style screens, but these are general issues, not 5mm specific issues.
One way I could see this being a problem is if the screen material is very thick. On a camera like a Rolleicord or 'flex without the interchangeable screen system, the screen is indexed to the top surface of the screen while the focusing surface is to the bottom side. So maybe the material is so thick that the viewing lens can't be adjusted enough to accommodate the thickness. This might be the 'Fresnel acting on the optical system' thing that the repair guy means? Or maybe the guy simply doesn't want to bother pulling the leather and front shroud to get to the adjustment screw to re-set the focus of the viewing lens to match the taking lens.
If you DO find that there is an optical issue, please post about it. I hear people mumble about this every now and then, but haven't heard a real explanation or a real case that can't be explained otherwise.
ic-racer, as I understand it the Mamiya screen is a complete replacement screen for the ground glass, not an add-on fresnel layer.