In my experience SVLAN maintenence on the exchange improved things muchly.
I went through sunday entirely without much of a slowdown...
But that might be because of the gateway I'm on, I have seen some perform worse than others...
But there was definitely SVLAN congestion as my exchange had 2 days of maintenence in the week that did indeed improve things at peak times.
Currently Plusnet "80/20" FTTC...Not overly impressed at the moment :|