Ahhh... sucked back in.. I couldn't resist.
I've heard it from PLENTY of Pianists, they just don't argue about it on internet forums like guitarists do. Bill Evans seems to rank up there with someone who has the best "Tone" or "touch" on the piano according to many of my fellow working piano players. My mom can't go down to guitar center and buy a 5150 and EVH guitar and suddenly sound like Eddie. I've heard the same thing said about drummers. Their tuning, touch and control totally affects the tone of the instrument. Vinnie C.'s Sidestick tone seems to be a signature tone that a lot of drummers I know seek. John Bonham's kick drum tone seems to be another big one.
I'm curious what defines "x" in your equation. Is it just the guitar, plectrum, cable pedals etc. fed into the amp or are there other properties that one skillfully develops?
I'm hearing something more along the lines of:
X = Things that do not change.
P = Things that are repeatable, quantifiable actions that a guitarist develops. Ie. Being able to consistently play a G major scale in time, in tune, at the same volume.
Ie. X (Guitar, pickups, pedals cables) and P (Pick attack, touch, Vibrato, etc). So the equation is (X + P) = Y
Because if thats the case. The Variable P, is subject to change due to the player's skill and choices etc. Therefore I don't see how Y can ALWAYS be constant if you change P (the player).
Just my 2 cents. This argument may come down to definition of terms.