recently changing the processing priority from the status dialog has been inconsistent (on Win7x64). The job is stated with the correct priority (as defined in settings, in my case 'Low'), but changing it doesn't have the expected effect. When switching it to 'Below Normal' (and checking with Process Explorer) the avs4x264mod.exe process is correctly changed to this ("Below normal: 6"), but the child process x264_64.exe which is actually doing the work is only changed to "Backgroud: 4 (Low I/O and memory priority)".
I couldn't figure out a pattern when x264_64.exe is changed to "Idle" and when to "Background" (or if that even makes a difference), but it doesn't seem to correlate with whatever you've set in the status window (and therefore what avs4x264mod.exe is set to) and for me it's never changed to anything else than those two.
(I'm on development update server, version is 2500, added a small log I had from a few days ago for system information)