It's not accurate all the time. It depends on many factors like other program using the bandwidth and stuff like that. Even your antivirus doing automatic update in background might give you a lower result.
Also when your ISP says you will have 8 Mbps speed they mention UPTO 8Mbps not EXACTLY 8 Mbps.
ISP's quote you theoretical speeds. that's basically down to the box around the corner.
and anyway, it's mostly to do with line noise/quality, not sheer speed. anything over 1MB/sec (around 100K/sec) will do for XBL. just make sure your line's solid and you don't live too far from the server or mirror.
that's MS's achilles heel - there isn't enough pipe i guess around the globe to make everyone happy.
When school (university) is in session, typical speeds are about 12Mbps down in the ResHalls. If for some reason I'm in the reshalls when school isn't in session, it can get up to 30Mbps. At home, I get 3Mbps. I don't experience network related lag at any of those places.