I have a 70 degree tank on my 570. It works very well. The OEM pump doesn't pull all of the fuel out of the OEM tank. When the 70 degree tank is added the problem is compounded some. I think that about 1/2 gal is unreachable with the setup as sold. I might try to make some changes so I can get more fuel out.
The rear vent hose runs up to the main tank. I pulled the ball out of my gas cap so both tanks breath. That has helped keep the fuel levels even in both tanks.
I am not sure if I will put one on my 390. It depends on how I use it.
Why would I want to do that??