I don't know anything about Web Tapestry at all.
However, some ISPs have a throughput profile which differs to the BT IP Profile.
Users on the up to 40Mb service tend to have a Plusnet profile of 37Mb.
When being switched to the 80 Mb service, Plusnet's profile can sometimes stick at 37Mb, so regardless of much higher sync speeds, the higher throughput is not achieved.
Plusnet have an "unlimited" speed profile that should be used for up to 80 Mb throughput.
This "unlimited" profile is strangely named 20Mb (don't ask).
I wonder if Web Tapestry have a similar setup, where the throughput profile also need uncapping to enable up to 80 Mb throughput speeds?