In my case I 'upgraded' from 20mbit to 50mbit. I was seeing 2400KB/sec speeds on 20mbit which was fine but after upgrading to 50mbit, I was seeing 600KB/sec speeds.
So for me the 'upgrade' resulted in a quarter of the original speed. I have since downgraded back to 20mbit because Virgin can't find any problems and that's exactly the same too. It's definitely not any of my kit either because my ADSL2+ connection quite happily downloads at 2000KB/sec and over.
Luckily, the speed issues I am experiencing are seemingly only affecting a small amount of users. Just be warned that if you find yourself in a similar situation, you're screwed because Virgin technical support actually said to me, and I quote, "I'm not sure what else to suggest".