I'm pretty sure that for musical practice, more practice (within reason) does help one improve more.

But another thing, that helps even more, is practicing intelligently, which means, directing your efforts mostly at what you can't do, and then, the rest of your efforts at keeping what you can do. You can play a sonata over and over, and get no better and keep messing up the hard bits every time. Or you learn how to break things down and focus on certain skills or tasks modularly, you can master them and then integrate them. I think that has a parallel to what Jay mentioned re: good writers always being able to tell you what they're working on these days.

(Horrible memories of trying to hit altissimo G, and altissimo Bb, on my old alto saxophone are flooding back horribly. Oh the horror.)

But anyway, from experience, I think there does come a point in musical practice where more practice just becomes useless. You get saturated and need a break. (It's also physically important to do so, as much as it is for writers.)