Cheers Mike,
Your modem will be synced at a higher speed which is a different parameter than the throughput which I think is what you are looking at.
For example, I live out in the wilds and my modem syncs at 4500Kbps and have a 3600Kbps service on a zero errored second count measured over several days. There are 2 cabs between me and the exchange so it's not bad in the circumstances.