I've never heard about the out of warranty perspective on TSB's. What I do know is that the dealer is not reqiured to perform the TSB if you cannot demonstrate the problem the TSB mentions. I had this problem with my Z. I was able to right after purchase to get one Nissan dealer to agree to replace my Bose amp. They were "ordering the parts" and would "call me when they came in". That was 2 1/2 years ago and I never received a call.
I took it into a different dealer a few months ago and discussed it with them. They tried but could not get the "volume problem" to occur so they wouldn't replace the amp. It hasn't had the problem in a year or more but I wanted to get it replaced if I could.