I don't have one of thsoe fancy watt thingies. But I'll be doing some prime95 and furmark testing on my CPU/GPU this week all inside the S4 mini, here's hoping there's no coil whine. I'll report if there is any though.
Don't get your hopes too high... my 1080 Mini has a lot of coil whine. But, if you download Rivatuner (it installs with MSI Afterburner) you can cap your frame limit to match your monitor and that will reduce most of it. I have the S4 close to me on the desk though. If you had it 10 feet away and/or use headphones it might be fine.
Let me know how you fare with it though. I think I'm gonna return mine and just for laughs try a 1080ti sticking way out of the case. It launches in 4 days (countdown on geforce.com saying "It's almost TIme" with the TI capitalized). I don't know if it's a paper launch or if there will actually be stock up for sale.