I found this interesting comment on Virgin forums about the outdated 'contention ratios'. It would appear there can be issues with sluggish speeds in certain circumstances.
"Virgin media is a little different from ADSL/VDSL ISPs though, and there is still some element of this at a local level.
Your coax cable will run past a certain number of houses, and the bandwidth on that cable is split between those houses. There are two ways to increase the bandwidth in cable broadband, either split the "segment" so that there are less houses per segment, or increase the number of channels available on that segment.
If you have 24 channels on your segment, then that's ~1200mbits of bandwidth. If you only have a hub 2.0, you will only be able to lock 8 of these channels, and should probably ask for an upgrade to a hub 3.0.
If that segment is shared between 10 houses, and all of them are on 350mbit, then there is a good chance you will feel contention (as there is only 120mbit of actual bandwidth each if you all rinse it at once).
If you're on 350m, but your 9 neighbours are on 50m, then you won't ever feel contention.
If you have 100 people on your segment, you will be more likely to have issues even if everyone else is on low speed services."