So, I installed a master filter directly to the phone line, where it comes into the house. Nothing else on the line. I am quite far from the exchange, right on the VDSL limit (if not a bit beyond). For both ADSL and VDSL the addition of the Master Filter made my throughput drop by about 10%. Which is OK as I just took it out again but I am curious if anyone has a reason why that happened. With all the reports here of master filters being next unicorns for throughput 'n all.
As FYI, the master splitter and modem was in a server cabinet so there would be a bit of noise. Just naked connection.
Or did I manage to hook it up incorrectly (I did try and check with Google)? I'd need to check again what exactly I did.