Would just adding a line like "This site uses AJAX for a large part of its functionality" work or would you prefer a more in-depth description?
I would want to know what I was getting in trade for the increased risk of enabling javascript. Your proposed message isn't substantially different from the current message.
Noscript users are going to be more savvy than average users, but they are still users so if you are going to make any effort to inform them, make sure it's from the perspective of a user rather than a developer. Developers care about AJAX, users only care about what they see on the screen.
But I'm a developer, so I don't know what the best wording for this is. The point is, some of the functionality of the site is broken when not using Javascript because it's AJAXed and there is no <form> fallback with POSTs and so forth (that's not in the development budget)
You charge extra to do it properly?
I only have so many hours in the day, I already got assigned on a different project, they told me they don't have time to support less than 5% of users.
Of course getting the explanation down is going to take some effort you don't want to be condescending and you do want to give a meaningful explanation - perhaps with pictures, animated gifs? - but don't want to make it into a dissertation either.