I mostly play guitar, but my experience is that weight does affect playing. 80-100 lbs. ago I usually sat with my left knee crossed over the right to play - now I can't can't even cross an ankle over a knee, and the change in position (added to the further reach around the guitar due to my belly) is, indeed, detrimental to my already fecal chops. Standing, esp. with my electric, I've had to move it higher, since if I wear it where I like it, it rocks on my belly - it was about Bobby Weir height, now it's more like B.B. That stabilizes it, but also angles it in a way I'm not really comfortable playing. I realize, of course, that all of this is within my power to reverse, but what can I say - I'm a complete food slut.
Peter
ps - In support of this, I would also point to the decline in Mr. Garcia's playing over the last 10 years/100 lbs. of his life. Though I would, of course, gladly surrender almost any body part not depending from my shoulders to play like his worst day.