In the meantime, I have given up on such adventures. The OM System OM-1 is slightly better than the Mk iii, especially what regards shadow noise, but that’s not the reason.
The reason is, that DxO Pure RAW 3 makes cleaning up noise a no-effort. It also makes ISO 1600 as usable as base ISO 200. Heck, even 6400 looks perfect with it. This means, I am now on Auto-ISO, increasing ISO whenever shutter speed falls under 1/5 s. As long as you shoot static subjects (I do), this makes for 100% tack-sharp images without any worry.