I've used my 240/420 Symmat converted on 5x7". Vertical format, maximum front drop on a Technika 5x7", f:32, no filter, FP4+ film.
The negative was as sharp as anything, and fully usable.
Using a strong coloured filter (e.g. colour separation) might be useful, but I saw no need for it.
I have also tried a 150/265 cunverted on colour slide (4x5"). There is some colour fringing at the edges, but better than getting no image!
Paul - "any filter (incl. UV filters and polarizers) will help cut the aberrant focusing" - that's wrong. A UV filter does nothing except introduce two more air/glass surfaces.
Claire - I own and use 180, 210, 240 and 300mm convertible Symmars, and have owned a 150mm as well. I also use them "converted".
Sandy - the front cell was originally intended to be usable as well, at f:14 and a slightly longer focal length. But it was always intended to be used behind the shutter.
A "Converted" cell will never be as good as a full lens, not even the fabled Protar Serie VIIa are as good as a full Serie VII. I know that, too, from experience...