Does adding additional Powerline networking adapters improve overall performance?

The monster powernet 200 documentation claims you should add more adapters to improve overall performance. I'm curious why they would state this if its clearly not true as the current answers state?

Their comment is quite subjective; if all of the powerline adapters were all electrically in the same location, then sure, adding more on the line would not only increase load, but add more noise filtering on the same line (hence creating a path of least resistance for the signal to travel over), thus adding more = a good thing.

Is it ever true? Is it just a gimmick to sell more units?

In your situation, I would say no; as your going across multiple floors. Given that you are traversing across multiple circuits, the information has to travel the length of the powerline adapters down through the circuit breakers to the other adapter in the house. Thus, a longer path than simply between both units.

Is it true only for some brands/models? (ie the powernet 200 also does some noise filtering -- maybe more filtering is better?) Maybe more powernet units helps the units better learn the noise characteristics of your power?

This really would only apply if each unit were on the same line; not split across multiple circuits. The current answer provides some additional detail as to why this would cause an issue; Houses with old wiring, for example, can interject so much noise on the line, or be ran improperly so much that increased attenuation on the line would cause the signal to degrade to the point where the powerline amplifiers couldn't properly pickup, amplify and filter out the noise.

Faulty wiring is another issue. Even if the wiring in the house is brand new, faulty wiring, short circuits, exposed wires, even wires going around other junctions or around other pipes that can cause interference can add increased attenuation and signal degradation to the line. Both of which the powerline adapter cannot fully compensate for.

So, the factors you have here are:

• Faulty Wiring: Injects noise and signal degradation
• Old wiring: Cause too much line attenuation to be useful
• Separate circuits/long circuit pathways: Causes increased signal attenuation on the line

And, lastly, you also have load:

When you add more units to the same line, you increase the load on the line; and as you increase the load on the line, you interject noise and attenuation; if it's on the same line, the attenuation can be compensated by the powerlines internal amplifier, but if it's on a separate line, it's like picking up a conversation in the next room, amplifying it, only to hear a muddy mess; unlike NCIS or CSI, these units cannot re filter out the signal and make the muddy mess into something clear enough to transmit to another unit.