I have a similar problem.
My ipad 2 was working just fine at HQ in my home network but the slingplayer on my 3rd gen which I bought a couple of days ago only gets SQ as a default and when I try to switch to HQ it barely gets around 630kbps and I cannot tolorate due to the frequent rebuffering.
I see no problem in either network or router and works just fine from PC (windows 7).
My assumption is the same that app is acting up.
Currently I have to watch it at SQ which doesn't make full use of Retina!!
I'm streaming remotely, but my iPad 3 is lucky if it cracks a 700kbps stream speed, so both hd and auto settings are not an option. SQ works, but I usually get better streaming speeds, at least enough to have it set on auto with a good picture while streaming remotely. Slingplayer for the iPhone app can run in auto mode and so far is functioning fine. Has to be an IPad issue.