Had a mark 1 faceplate when I first had BT Infinity 2 (18 months ago). Speed was approximately 62Mbps and with crosstalk eventually went down to about 52Mbps.
We had a near lightening strike in the summer which killed the Mk 1 faceplate. As a temporary measure I replaced it with Adsl-nation XTE 2005, the down speed remained approximately 52Mbps with a latency of about 10ms.
I replaced the XTE 2005 with an Openreach Mk3 version. My speed was reduced from 52 down to 41Mpbs and never recovered above 42. Latency stayed at the same 10ms.
As an experiment I replaced the Mk3 with a Mk2 version, and my speed immediately increased to 50 again with latency staying at 10ms. Buying the latest Mk3 faceplate crippled my download speed.
I've placed orders for a Mk 1 and Mk2 as spares!