I have a 4x5 (9x12cm) "Messraster" focusing device in a bought-new two year old Sinar P2 (see
www.anstendig.org, the papers on focusing, for info). This rarest and most accurate of all focusing devices (it is probably the only large format Messraster still in existence) is mounted in a Linhof back that was adapted for the SinarP2's back by an East Bay shop, which was the only such place I could find with the precision and capability to do the job (Adolph Gasser Jr was sympathetic, but didn't want to touch the job). The back was originally mounted for me (when I lived in Berlin) in a rare French ILKO 5x7 camera's back...which camera I still own. It was the most advanced camera of its day, with a lens-standard suspension system that absorbs shocks and allows total movements, etc. Quite a device, but parts or accessories or lens boards available anymore.
Focusing with the Messraster is absolute. It is very easy with faster lenses and takes just little bit more time with slower lenses, unless you have excellent young eyes, which I no longer have. A Messraster with slightly greater distance between surfaces would be easier to use with slower lenses.
While this camera is fun when one has the energy to schlepp it around....or a few students to do that for you, I find myself almost not using it at all, because the new EOS-MK II with shift lenses, etc., can get most needs done much easier with adequate resolution......provided one uses only single fousing sensors and places them exactly on the most important image point/place. There are, however, some subjects that even that autofocusing system cannot manage, and which the Messraster can. So I still keep my old Nikon 35mm camera with messraster and the Sinar handy. High res digital backs for LF seem to have large-magnification focusing possibilities. But they are not all that handy, which the messraster is.
Unfortunately, there is no cheap way to do accurate photo work with mid sized format. Second hand I am leery of, as the cameras are seldom well-adjusted depth-wise even when new. And the problem of mirror slap and such with midformat slrs daunts me. Even a tripod is no assurence of lack of movement.
Going to larger formats has its drawbacks, too. With film, there are very few cut film holders with accurate depth....I haven't found any, except the ones that were adjusted for me for depth by the Messraster inventor back then. Maybe the super-high-priced highest end Sinar holders might be exact enough. But I wouldn't bet on it. Then there is film thickness, and film flatness. Big issues. So yes. there are quality improvements with larger formats, but less than proportional to the larger size. Far less.
I have Linhof Ideal format shots that rival, in detail and everything else, 8 x 10 and larger formats, because the film lies flatter, the emulsions are thinner and better, and the lenses for that size format are generally sharper.
Mark