I realise they are rare, but when you get better results w/out it, and nowt else changed (no wires), it's worth considering. Could be dirty/damaged contacts.
At the mo' we only have one trial. I think it is worth the OP trying more trials of with and w/out the faceplate (but not too frequently to cause DLM to react) and see if there is a consensus of opinion.
Of course these speeds are v. low anyway so their diff might be insignificant.
OTOH if speed is always about 200 K with faceplate and 700 K w/out faceplate, then it should make you wonder.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC