My Netflix app on the Roku has a sort of "nerd view", where it'll show the bandwidth of the stream that it's able to get.
And, at 15-20Mbps, you should have no trouble getting 4K quality video, that's plenty of bandwidth for that. There's a real perception that households need 50Mbps+ just to limp along, and that's just not the case. (I'll posit that this may be because consumers have been buying faster and faster headline speeds on Cable, which then delivers a congested experience in the evenings, or congested interconnection to the sites consumers are streaming from. YMMV.) Typical HD video uses just 4Mbps, and 4K is generally 9-12Mbps. This makes a solid and dedicated DSL line that doesn't have neighborhood evening-time usage congesting it a more consistent experience.
Another potential to consider is WiFi. If you're using WiFi to connect that TV or streaming box, I wonder about capacity there. We're now offering members a really great whole-home WiFi solution that is worth considering if the speed of the internet connection is good but usability around the home varies.
But on your HD set, you won't be getting 4K, so I'm not sure why this content would look any better than any other HD content.
Regarding the Pace, if it's a Sonic branded device try visiting http://gateway.sonic.net/
to see how fast the connection is.