Well for what it's worth, I'm on a profile of 7150 and get usually get around 6.1 to 6.2 Meg. So if we chop a similar amount off your profile, you should get between 4.0 to 4.1 as a ballpark figure.
It might be worth trying out the TBB latency tool:
http://www.thinkbroadband.com/ping
You can use it to get an idea of if you latency is solid or jumpy, even when idling (so to speak). Per the example pic in the link, you'll see the lowest latency is in green. Generally my latency to TBB is low 40s.
You can also check out the latency to the DNS servers. Download this little file I use:
www.camieabz.co.uk/batfiles/pingdns.bat
If not familiar with batch files, run a dos command window, see the default path, and drop the file to said path, then just type "pingdns" in the dos window.
That should show you if there are any massive differences between the Plusnet's DNS servers for you. Then it's a case of changing the default preferred serves in Windows and / or your router config.
Generally I find most of any slowing down problems are contention or latency. There's a couple of other little tricks to speed things up, but you need to ID if it's latency or not first.