I honesty don't think the 'thinkbroadband' site / server is up to the job for those with upper tier connections. I've had to stop customer's from using their speedtest site in the past as the results were inconsistent and not a true representation of the state of their connection.
If you are seeing the full speed on speedtest.net and on bbmax I really wouldn't worry - your connection is working just fine.
There are an awful lot of things that can affect a file download and in that case I think their server is a waste of space.
Try downloading a large file from a decent server - something like a linux OS, ubuntu's servers used to be pretty fast - haven't tried them recently though.
Rich