An update to my previous post, but dropping priority to "Normal" mode.
General system responsiveness is normal. Handbrake's average frames per second county thingy now goes to ~8.1 on average with extremes being more like 7.7~9.3, but it pretty well hovers at 8.1.
By contrast, applications like writing this post in a browser are now normal performance. It's even possible to watch previous encode without glitches and artifacts in the decode. But things like writing this post cause more dips in Handbrake's frame rate.
Overall, it seems fair to say "High" -> "Above Normal" priority loses 5% if you want a nice round number instead of a range for the average. "High" -> "Normal" priority loses 15% by the same method.
Where "High" = cripples the desktop session, "Above Normal" = makes the desktop session feel a few hardware generations behind, and "Normal" = makes it just another process.
I guess that solves that, lol.