I'm not sure what type of quality you are looking for. It's not HD, not 720P, it's viewable. You keep talking about upload, which is important, however do you even know the Kbps needed for slinging? I could be mistaken but if I remember its 500. Video looks fine at lower than 350. Do you even own a sling 'cause from the way your talking you sound like a theorist, not a user.
I've owned and used a SlingBox Solo for over two years and I've watched video from that on both my iPhone and MacBook (receiving over WiFi). It would be a pretty big stretch to call the quality good when sent over my 350Kbs+ upload speeds and on the MacBook I usually need to lower the frame size and frame rate and total bitrate to the point were the video is blurry and somewhat choppy. On the iPhone I always see dropped frames and notable pauses in the playback.
However, the same is true for my EyeTV when it is "throttled" though my ISP's slow upload speeds. In any case, when I use my iPhone or MacBook from within my home's local WiFi network the video quality is actually very good (because in this case the video transfer is contained within my local network and the speeds are in the tens of megabits per second range).
I've also used my EyeTV Plus over 3G using the EyeTV Live3G website. The live video looks okay (with a good 3G connection and the EyeTV server running on my Mac Pro) but it also pauses continually (plays for 20 seconds, pauses for 5 seconds, then repeats, etc.). The only way I can get uninterrupted playback is to compress the video to EyeTV's iPhone Cell 3gp setting which is a pretty lousy 176x132, 15fps, 84Kbps video stream (which is kind of ashamed because EyeTV doesn't allow you to adjust its 3gp setting and something in the 200Kbps range would probably look okay but certainly not great).
Note that I'm not saying that you can't get good quality over the SlingBox, but you need something better than Road Runner's Basic or Power Boost service. A really
solid 500Kbps might be good enough but I've never seen uninterrupted (pause free) good quality over 350Kbps.
In any case, here are some bandwidth results from my Road Runner service here in Cary, NC (using SpeedTest.net):
Mac Pro (two trials):
ping: 33ms/68ms
download: 9Mbps/11.6Mbps
upload: 360Kbps/360Kbps
Here are results for my iPhone 3GS using the SpeedTest app over
3G (from my home, two trials):
download: 1.5Mbps/1.9Mbps
upload: 213Kbps/232Kbps
Thus, you can see from these results that the bottleneck is the upload speed on my Road Runner cable service (because the video is sent through the 360Kbps upload link and received on my iPhone's 1.5Mbps+ download link).