Hi, firstly please don't post multiple times about same issue, you won't get help any faster and it impacts other posters getting help and can lead to your posts flagging as spam.
Secondly, I will try and help you with your issues, staff from the team will not get round to this post for up to 2 weeks.
When transferring data (such as uploading) it used Megabytes per second to measure not Megabits as such will appear like as MB/s or Mb/s (so 50Mbps is around 6.2MB/s)
Speeds can very for different reasons, the reason for the computer not processing more then 300-400mbps of your download could be related to computers specs (this includes available memory, CPU performance and hard drive performance) positing computer specs could help us cross that in out of this situation.
I noticed in your other post that you quoted speeds using a network extender rated 500mbps (these rating are purely up to and are affected by all sorts of things such as house wiring if power line adapters, the distance between them for both power line and wireless adapters) generally I've never seen these adapters perform much above 100mbps and most of the time that is because they only have an Ethernet connection of up to 100mbps.
Unfortunately due to the nature of Gig1's service if its testing OK to the Hub you won't really get much assistance from Virgin's support.
I'm happy to help you go through some troubleshooting and check some particulars with the connection to make sure all is as it should be.