I just put my tele right onto the UV filter, sometimes I add my polarizer into the setup as well. I am using a TCON 14b. I do not notice any additional vignetting (to what the tele itself creates). This has been asked here before and I am guessing you will receive comments suggesting to leave it on or remove it.I basically leave my UV on all the time and then add any lens or filters on top of it. The threads on my TCON are damaged & nonfunctional but toting around two sizes of filters is a bit expensive and impractical.
I've been removing my UV filter. It's been my understanding that the closer the teleconverter is to the Leica the sharper the picture. Have you experimented to see if there is any difference with and without the UV filter? I would be interested in your findings. I haven't done that test yet.