Ill just start by saying I'm on the 4mb package and my speed is always hovering around 3850kbps to 4mb, so I'm very happy[obviously] with the service.

I want to upgrade to 20mb, but whats worrying me are all the stories I read about people saying they don't get anywhere near it, in some cases saying they get about 1mb. What I want to know is that if my speed has been reliable on the 4mb service, will it follow that I should be OK on 20mb.? I'm guessing I get a reliable speed due to the fact that there are only 18 people on my "green box" at the end of the street.
PS What is the correct term for the "green box" please.