I imagined that the FTTC service was on different frequencies, but the ADSL2+ signal would be boosted / repeated due to the cab.
Nope. The two are independent. You could blow the FTTC cab up and it would have no impact on ADSL connections (unless you used so much explosive that the original cab was caught in the blast). FTTC connections are made by patching your telephone line over to the cab and back. If you have ADSL there is no such patch and your telephone line continues back to the exchange like it always done(*).
VDSL signals probably do have some impact on your connection speed but it'll be in the form of crosstalk and is more likely to hinder than help. ADSL and VDSL use the same frequencies. The main reason VDSL is faster is because it uses a greater range of frequencies. There's a bit more to it than that but they are basically the same technology.
(*)For FTTC the line also goes back to the exchange but it first passes through the new cab and that removes everything except voice signals.
Edited by Andrue (Tue 04-Mar-14 13:36:43)