800m is quite a lengthy D-side - only 20% of lines are longer. But your neighbour's speed is more realistic than your estimate.
See the distance table on here (speeds are pessimistic, so should represent the bottom end of what is normally possible)
However, you have to remember that the gauge and material make a difference - so if your line is of thinner copper, or even aluminium, then you could be doing worse.
But BT have been known to get the estimates wrong in some places too.