I read an article perhaps 6 or 8 months ago about Les Baer, and the difficulties he was having keeping his long slides within tolerances; it seems that that extra inch of slide length really does make it very much harder to machine correctly. It seems that he has overcome these problems, however, so I would probably go with Les Baer making me a custom pistol, if I really wanted a long slide.
I feel confident that a master craftsman like Les, or his fellow smiths could handle such a project even in the dubious utility, near moribund, poorly regarded by rational people 10mm. caliber.
Walt