I think this sort of thing probably works best as a motivational tool. It just reminds you when you need it that it is something you love, and something that you love to improve in. When you can't be bothered getting up and going for a run, it reminds you that all the greats would have jumped up to go for a run in the beleif that it will make them better.

I also feel it would do the same for beginners right through to the experts.. If you notice one thing about the top pro's, it's that they do not EVER have any doubt in themselves. and if they do, they NEVER show it. Sometimes even after that have been destroyed by a better boxer... And as I said in my earlier posts, the pro's get millions of dollars to act, feel, and BE the best... At most or our levels, when only get enjoyment from it.. And sometimes our commitment or heart just isn't in being the best we can be. For whatever reason... This just re-enforces it in our minds...
It by no means makes you a great boxer. You still need to take every step in the gym the same as the next guy. With or without these affirmations , anyone who put in the time and effort will become a good boxer...

I just feel that Beleiving you already ARE a great boxer takes you one step closer. But as someone else said, it may not be for everyone, and it's up to you to decide if it would benifit you or not.