I have a Billion router that I use with the BT-supplied FTTC modem, wired Ethernet joining the two.
advantages of that configuration include...
1) The FTTC modem is in it's optimum location beside the master socket which, usually and for me, is on an outside wall.
2) The Wifi AP is in optimum location, centrally in the building and at raised elevation.
These advantages do not affect sync speed or error rates. I would expect there to be bo difference whatsoever vs a combined modem/router located by the master socket.
I don't remember exactly what was involved in getting my billion 7800 DXL (iirc) to work with FTTC. I do remember I was entirely confident that having researched the issue, and set it all up it in advance, it would 'just work' on the big day when I switched from adsl. In fact, it didn't 'just work' . But overcoming my embarrassment, a restart and allowing the wizard to set it up for me worked perfectly.