You are forgetting about the transmission protocol overheads, they total approximately 12.9%.. So the most you are going to get is around 87.1% of your sync speed.
Is that the same in both directions? 12.9% seems wrong at least for downloads - people report actually achieving 76Mbits/s on speedtest.net on a 79.999 sync (95% of sync speed).
EDIT: I've now unlocked my modem and disabled QoS. I now know that it's syncing at 79.999/20.000 (max achievable is 87.948/32.506). Here's a typical speedtest:
Yet again can't get BT speedtester working, But it was my understanding that everybodys IP Profile on 80/20Mb was set at 78.33Mb D/L and 20Mb U/L or am I wrong again