I've tested similarly on a Roku LT and the bandwidth utilization always goes to 5032 Kbps. It never goes above that, and it stays at a 720 resolution. For me, I don't see it as being "soft" at 720 resolution, rather I notice that it appears that frames are being skipped. It doesn't look like the video is as smooth as it should be.
Meanwhile, other apps like youtube and such on the Roku always appear very smooth and with a decent refresh rate. I posted another post similar to yours a couple of days ago, I hope someone at sling responds with a fix, or they maybe fix it from the perspective of these video streaming player apps.