The effect of shortening filister screws is that allows the magnetic flow to be sent to the strings being slightly stronger.
Per se, this doesn't brighten the sound, but it definitely helps playing nuances based on touch-sensitivity and pick attack come across strongly.
I first set the filister screws stagger to even out the output differences between strings, so they basically look like this:
... THEN I cut'em flush with the baseplate. A MUST for neck p'ups, not so much a life-or-death issue on bridge p'ups, if you ask for my personal opinion based on over fifteen years of first-hand experience acquired in the performing of well over a thousand p'up swaps, mods, sweet-spot setting and general guitar setup optimized for both live usage and studio recording.
/Peter