Re: painting my guitar
In my opinion, it would make a difference in theory. However, in all practical uses, I don't think you'd be able to hear it.
I did my final year engineering thesis on acoustic guitars - comparing 2 guitars (a taylor and a yamaha) and carrying out vibration analysis on the guitar top. Although I didn't analyse the results of different finishes, I believe that the type of wood and the bracing influences the vibration of the guitar so much more than these smaller factors.
And think about it, when you're playing, you're holding the guitar against your body... so you're already damping a lot of the vibrating surfaces. And if you're amplifying it, the pickup, amp, EQ, etc will largely overshadow other factors. There are so many variables and the finish is just one of them.
So in short - Yeah there's gonna be a difference but I doubt we could detect it in a blind test.