"in a hypothetical situation I would end up with a better service"
yes, you may as you will have to be issued a new DS3 modem, but more important, moved to a UBR that is DS3 50Mbit card enabled, assuming your house/area is currently active and taking DocSis3 customers right now OC....
in effect the DS3 is a virtually seperate network on a different fibre light source going down the same wire so your not likely to be over subscribed etc as yet on there.....
now paying for the hidden £50 11n wireless router fee + £30 re-activation fee =total £80 for the official VM DS3 installation fee is another matter all together OC