Now about the NTE5 box? What would this do?
How would it improve performance?
The NTE5, with a filtered faceplate fitted, isolates all your other extensions from the ADSL signal. The wiring in extensions sometimes causes interference which results in noise on the line, thus reducing your sync speed. The other extensions become incapable of receiving an ADSL signal so you must use the filtered faceplate. Loose plug in filters are not then required on the extensions.

As you can see below Iím not getting anything like 6MB
Unfortunately some Sky agents refer to the sync speed as the download speed. This is incorrect, your actual download speed will be between 85-90% of you sync sped, so you are doing OK.

With just one extension you may not get much advantage out of fitting a filtered faceplate. It is a bit extreme, but the only way you can check what improvement you would get, is to remove the wires in the Master socket which lead to the extension. This would have the same effect as using a Test socket. Don't do this unless you have an IDC tool to replace them. You can get one in places like B&Q for about a pound.