How about upstream and downstream on status page of W9980?
Can you provide those info on your HH5 as well?
What's the current firmware version?
Did you try to download something online and check the speed?

Thank you for your post.
My query is not in any way a criticism of my TP-Link. It's mine, it's stable and I love it, so there:D
When I upgraded the firmware to Build 150507 Rel.40130n the notes included the following.

1.Fixed the bug that the speed dropped on the BT line of UK when G.INP was enabled.

Now, I could have assumed that this referred to the cabinet, but I did not, I asked instead.
I also asked because of the download speed difference, just in case.

To give some background/history that may, also, be of use to others.
To begin with I had a HH3 + O/R modem. This gave the same stats as my TP-Link does.

TP-Link
Upstream Downstream
Current Rate (Kbps) 6963 34998
Max Rate (Kbps) 6673 40993
SNR Margin (dB) 5.3 8.8
Line Attenuation (dB) 28.2 26.1
Errors (Pkts) 0 0

Then, due to issues, I got a HH5 and another and another. Every time an engineer came out he tested the
line, said it was brilliant and gave me a new HH.

The stats for the HH5 as follows.
Data Rate: 6990 / 40000. Noise Margin: 5.7 / 8.1

Now bear in mind that the line length is 500M to the cabinet (tested), how does the HH5 get 40Mb download.
Using old hydraulic terms I would expect a pressure drop at the hub to give the increased flow!
It's almost as if the BT HH5 is saying that the 'copper line' does not have an effect on speed!

Your last question is - yes. speedtest.net (flash) and speedof.me (HTML5 in browser)

David