I wrote a script to manually control fan speeds on headless linux boxes.  It's adapted from another source online, but modified for newer drivers.

https://gist.github.com/squadbox/e5b5f7bcd86259d627ed

The script can be called after using the coolbits flag as dominuspro mentioned a couple of posts ago to set up X.

You could use the same format and adapt it to manually Over/Underclock or Over/Undervolt cards on linux using:
where the number in square brackets is the performance level.

I haven't yet used it to adjust OC.