Just realized OLD thread. But I'll leave the info anyway.
AFIK, Sling tech only requires an average of about 7.5Mbs for the highest quality HD. This is my experience from my Slingbox. So, 6.5Mbps is EXCELLENT FULL bandwidth for the very best HD quality. The bit rate will vary and is dynamic because the less video information (such as talking heads or very steady scene) the less bandwidth (even as low as 3.5Mbs) is required to maintain full BEST HD, while a scene with action or many elements will require more bandwidth to possibly maximum between 6.5-7.5Mbs to maintain Best HD.
Sling proprietary re-encoding technology allows for a FULL 1920 X 1080 using far LESS bit rate than other competing products, and can also change on the fly by increasing or decreasing based upon your connection speed without stuttering or pixelating, but turning soft or sharp, instead. so that if your ISP is suddenly only to provide 4Mbps upstream, you can still get a GREAT quality picture that automatically adjusts. Sling can send excellent quality PQ at even 3Mbs while competitors greatly struggle with this and have really bad PQ with limited bandwidth.
My experience with my Slingbox on my LAN to both my PC and TV (via WD device) for BEST HD (1080) varies between 3Mbps to 7.3Mbps depending upon the scene. It most often hovers about 6Mbps with it only rarely exceeding 7Mpbs because of a really busy scene of video. I have to say that the picture is SUPERB and on the HDTV, one can NOT tell the difference between live HD and the HD being sent by my Slingbox to my WD connected to the HDTV. It looks as if NO LOSS (but of course there is with re-encoding anything) PQ, Sling is that GOOD.
So, you are doing as designed with the FULL Best HD possible. It's just that Sling proprietary re-encoding can send an astounding PQ full 1920 X1080 Best HD using a bit rate around 6Mbps.