It certainly does hold water. I'm not saying that the only thing you need to do to get a 3800 to sound decent is make both secondaries the same length. There is obviously more to it than that. But it just seems as if 999 out of 1,000 exhaust threads here revolve around cat or no cat, resonator or no resonator, muffler or no muffler, which muffler to use, which cat to use, blah blah, etc. etc. and almost no one addresses the giant elephant in the garage, which is the front bank travels significantly farther before it joins the rear bank. As you've said, because of it's funky firing order and split journal issues, these engines are already behind the eight ball trying to make them not sound like crap, and the grossly unequal secondaries just makes it harder. So it does matter. It is a piece of the puzzle. That's all I'm saying.
A V8 is a V8. It's always going to have certain basic sound charecteristics, no matter how you exhaust it. But listen to sound clips of the same V8, one with equal length secondaries and one with a FWD, crossover type setup, and you will hear a difference. The unequal length crossover enhances the undesirable half-octave tones and some rasp.
When I said turbo 3800's sound better than non turbo cars, I was referring to the "newer" series 2 and 3 3800 kits that have closer to equal length inlets, as opposed to the old Grand National turbo setup, which has very unequal length inlets. The Grand Nationals still have the accentuated half-octave tone, but it's masked somewhat by a different cam than we have and of course, the turbo.
The closer to equal length your exhaust secondaries are, the better the pulses "line up" to exit the pipes, and the "cleaner" your sound is likely to be.
