Sling player mobile works with WM6 since like 2 days before the Mogul came out. I tested the BETA
I usually use the High settings. If you are having issues and you're running WM6 and the Mogul on Sprint, than I am betting your home DSL/Cable network has crappy upload speeds or are using it for other services not allowing Sling to flourish and you're not getting the desired results. I use Comcast and get 784 upload so I can throttle it pretty hard.
Also make sure you have the latest Firmware on your Slingbox.