I just received a Dell D6000 to connect my three monitors (2 x 1440p using DP, 1 x 1080p using HDMI) all running at 60hz and it does reduce the overall power consumption by 10W, making the fans runs at 2300 RPM while playing a 1080p video on one screen, which is virtually silent in my case. Macbook lid is closed.
However the 2 display port screens lag one frame behind the HDMI one and I noticed it right away while moving the mouse. I'm not sure if I can get used to it. I just bought this 200$ dock to eliminate the fan noise but I feel like I just downgraded my setup. I also feel like the refresh rate is below 60hz on these screens when I move some windows around.
By the way, I think the HDMI plugged in the D6000 act just like it was plugged directly on the Macbook. It consumes 7W on the dGPU even thought it's plugged in the dock, and if I open the lid, I get the "standard" 18W as well. Also the HDMI screen works without starting the DisplayLink app while the DP ones dont.
Pretty sure I'm returning the dock, and maybe the whole macbook as well, this is so frustrating.