I haven't noticed any pq problems and the picture quality can be set as high as 1080 at 10mpbs. I use the recommended 720p at 5mpbs. If you use the higher 10 mpbs rate it will limit the amount of tuners you can use at a time. I think the 4K output of the Roku makes the pq look very sharp and better than just ota hd.
HERE are the settings you can set your Dvr to, on both live tv and on recordings.
720p 3mbps
720p 5mbps
1080 8mpbs
1080 10mbps,720 @60fps
Do you have guide data for your subchannels?