Maybe you can run VaM in Fullscreen mode to have 1920*1080.
(Start VaM with VaM (Config).bat or create your own bat file)
My 5800x is running with PBO2 on(PPT85 TDC60 EDC90) but still gets 4.8GHz max so I think CPU under clock doesn't effect much to the result.
(CPU usage was 25 - 30% during...