just wanted to say that this worked beyond my wildest dreams.
i took a chance on a logitech g502 x lightspeed thinking the 'configure extra buttons' thing in the plasma settings would do the trick. it did not. nothing.
but piper/libratbag did, massive success! and such a simple easy to use piece of software too. needs a wired connection to set it up though.
i still have nightmares about such software in windows
now lets hope logitech have improved since last time i used their (then shit) hardware about 5 years ago