Something like the ipower will not help if you have noisy mains and/or bad grounds (earth connection). Neither will a conventional linear supply. For that you would need a large isolation transformer($$$), and/or large line filter($$) inline with the mains before the power supply. Or that battery bank (least expensive).
None of that including a battery bank will stop noise from the ethernet cables or (in my case) the HDMI cable.
As you said yourself, ethernet "might be" a source of noise, but it does not mean it "will be" a source of significant noise in your case (or anybody's). For me personally, "might be" is not a reason to spend several hudred dollars on power supplies. Especially since a WiFi dongle can fix that for $10.
With just a single connection to an audio system (WiFi instead of wired ethernet, no HDMI, no powered USB hub), and a stock 2A supply, my Pi2/DAC+ combination has very low noise, and extremely low distortion (also assuming volume control disabled), even with poor mains power.
In the test above, with a less-than-perfect 1KHz test signal, 48K sample rate, 16-bit resolution, harmonics peak at -100db and noise is down around -110. That is almost studio quality sound.
When I start adding other things, and other connections, the noise goes up, and eventually becomes audible. No power supply can prevent that.
Neither a linear supply nor a better smps will prevent noise from the ethernet cable, or the HDMI cable (something I recently found out while testing).
Don't get me wrong, I am a big believer in clean power, and I used to design and test power supplies as part of my job. I could go on and on about the benefits of good power supplies in audio, but the fact is that you get most of the same benefits from simple and cheap solutions.
