I recently got a TP Link W9970 after using the Plusnet Hub One. The Hub One was syncing at 9999/55000.
When I first started using the TP Link it sync'd at the same speed and was working ok. It show a higher max rate than the hub one.
However after 2 days I noticed that it had re-synced at around 5am and the downstream sync speed was much lower around 48000. The next day the same thing happened and it dropped again to 46000.
At this point I swapped over to the Hub One and after a couple of days the sync speed started to increase. I decided to try the TP Link again. The sync speeds went from 55000 to 49000 to 44000, each time re-syncing at around 5am.
I think it is quite common for third party modems to sync lower than the BT HH5 / PlusNet Hub One - I saw something similar when I changed to an ASUS DSL-AC68U however I was able to recover most of the sync speed by enabling G.INP in the router settings and also dropping the target noise margin to 5db. Enabling G.INP also seemed to switch my line from interleaving back to FastPath with much better ping times.