Even though it is a Cat5e cable (with RJ11), it will still lose more used as master socket to modem extension cable than exactly the same cable (with RJ45) would as an ethernet cable between the modem and the router.
That loss may be a worthwhile tradeoff depending on other considerations. I don't have a feel for the expected loss as a modem extension cable; there should be effectively no loss if you use ethernet to separate modem and router.
Moved (with trepidation) to BT Infinity 2 for upload speed. Happy BE user for several years.