RR,
It seems to me that the benefit of the ADSLNation faceplate is that, when the router is plugged directly into it, it can be wired so as to isolate the DSL signal pretty much 100% from the house wiring, which must be a very good thing. But that does depend on plugging the router directly into it, and I think you've said that's not an option.
If the router's to be on an extension, I think you'd need to wire the ADSLNation faceoplate so that the unfiltered DSL is carried to the extensions. Looking at the installation .pdf that does seem to be possible, but I can't then see that it would be any benefit at all, other than the spurious effects already discussed of extra filters.
That said, I'm no authority on filtered faceplates. I've never tried one because, like you, my router has good reasons for being on an extension socket, not the master. If I'm missing something, or even if I'm just talking blatant rubbish, I'm sure somebody will set me to rights...
- 7LM