Hey folks, I gave THREE pieces of information based on what I had read so far, with 'early '06s' being just one of them... and the least important.
What I'd do if I was an '06 owner, Yamaha letter or not, is to follow my 3rd recommendation: ride the darn bike to a mountain and see if I have the problem WHILE CLIMBING SLOWLY (remember you won't feel symptoms riding aggressively), but CO settings of 5,18,18,21 is a good indication you will.
If you have the symptoms, get the ECU replaced. If not, leave it alone. But DON'T assume you don't have the symptoms without riding the bike to a mountain. Even if you don't plan to ever use the bike on a mountain road, you better find out if you have it or not for resale purposes, or just get the ECU replaced (if you have a letter) and ask dealer to hold old one for a day in case you have rideability problems.
Remember boards like these are just OPINIONS, including all mine, without having to write 'IMO' every freaking paragraph. I'd never take any opinion, even from the 'experts' for granted, but there's a wealth of very useful info on most boards like this which helps to make our own decisions, and find solutions to our problems a lot easier. Remember a dealer will never accept as fact 'I read it on the internet', regardless of the poster's credentials. Have a great weekend gang.
JC