I think you will find that TT are looking at the usually very conservative BT estimator. What does the BT Retail estimator give, and also the
BT Wholesale one.
Putting your stats into one of the best estimators, and assuming from your opening post you connect at 8128kbps, then a connection speed of around 20Mbps with a download speed of over 16Mbps should be possible.
There is a slight possibility that Turbo Boost or whatever TT call it isn't available on your exchange which may be why they give the 8Mbps or so figure. I only found out recently that not all TT exchanges supply it.
I suggest you ask in the TT forum about service packs and MSDN ISOs. Sounds OK to me, but I'm not a TT expert on that aspect.