for my units it is certainly true. For example, my main PC in my home office connects to my BT homehub via 200Mbps homeplugs. If I plug my homeplug into the same socket as all the PC gear is hanging off-of, i.e. PC, 2 monitors, several disks, NAS, several printers and scanners, phone charger, battery chargers (a frightening tally actually) then I get something like 50Mbps or sometime nothing at all, even if all the kit is in standby. However, If i plug into another wall socket (or unplug all the other c**p) then I get something like 150Mbps.
I believe it has something to do with the number of transformers hang off of the local circuit introducing local noise. Especially the "slab type" external power supplies that are on all the time regardless of if the equipment is in standby or not.
Another example, The home plug serving the Hummy will vary it's connection speed considerably if I unplug the Wii. Even if it is standby, it's transformer seems quite noisy..
So I think in many cases, you'll get a good connection. But your mileage varies depending on local wiring and what else you have plug in the same outlet.
For me though, anything around or above 100Mbps is fine.