I can confirm that some brands of plugs definitely work best when on the same ring-main. In my case that means upstairs to downstairs connections have much lower speeds than same-floor connections. This can vary a lot according to layout, which sockets are used, other devices plugged in, etc., but it's worth checking before making a large investment.
Also, if you are a keen radio listener, some reduction in quality of radio reception (AM and FM mostly, and shortwave if that's relevant to you) may be noticed since the data signals can be radiated by your mains wiring and can act as a source of interference. DAB radio reception is less susceptible to this (but not immune).