Have been trawling the forum for people with similar issues..
I have 2x FTTC connections and third FTTC connection across a PTP Link all bonded.
When the 2 local FTTC connections are bonded with similar latency i get the correct throughput.
The problem comes when i bond the third (Slightly Higher Latency) conenction the performance drops down for single tcp transfers.
All very strange.