I have never ever in all the years we have been doing this seen a plug in filter cause any kind of drop off due to its lead. With regards to extension wiring, again, if the cable is good quality(CW1308 or CAT5) then there will be little or zero difference from the test socket (providing bell wire filtering and or/centralised ADSL filtering is taken into account). Every installation is different but in the majority of cases extensions are no problem if done properly.

Plug in filters do not isolate extensions that may not require the ADSL signal from the bell wire so it's always better to go with centralised filtering via a faceplate if possible.