Haven't changed the plug but I've had it out and cleaned it up a couple of times. Perhaps I'll change it and see how it goes. As you say, if it fires up straight away, and it usually does, there's no problem. The ferry incident was disconcerting however, and I'm wary of switching off now when I'm out and about.
As to engine noise, it does vary but I'd say it's noisier when warmed up. I usually put it down to how much I'm focussing on it, but I generally notice it because I notice it, not because I was listening for it. I'm also aware that for the first weeks I was fully focused on staying on the bike and noise wasn't registering much. But as you say, it runs fine so it's probably ok. I'm thinking earplugs are a very good idea. First long trip with the new pipe the other day I stopped on the way to buy some in fact. The helmet helps of course, but I suspect the noise level is above what's desirable.
So is your dealer going to replace the auto decompressor - or is it wait and see ?