I've played guitar as a serious hobby for 32 years, speedcubing for under a year.
Instruments that impose otherwise unnatural fingerings help develop full finger independence which can definitely carry over to cubing. Not necessarily the muscle memory you already have but the ability to eventually impose your will when it comes to doing naturally awkward things. I'm very slow by speedcubing standards but I think my turn speed is above average for a mere mortal pushing 50 with as little time as I have under my belt. I also have confidence in improvement from decades of guitar experience. The discipline has taught me not to accept current limitations and plateaus as well as how to systematically attack those physical roadblocks: patience, repetition, working on difficult passages slowly, repetition, learning to relax rather than tense up, etc. I know I will be able to turn significantly faster in two years, barring injury or the onset of arthritis.
My technical playing experience doesn't help me with lookahead and recognition, however, and those will almost certainly determine my overall potential for speedsolves. I think someone with a dexterity deficit but good visualization for the cube will improve more quickly than I.
I think there are numerous correlations between the two beyond dexterity. As Ian Scheffler also mentions in Cracking the Cube, algs are like scales, working out a solve via a particular method involves improvisation. That concentrated state you can reach feels much the same to me, at its best it almost feels like you're a spectator to what you're doing. Athletes and gamers get this too, when baseball hitters talk about "seeing the ball well" or any mention of being "in the zone". Those moments where time feels dilated and the world around you seems to be in slow motion.