I have been in the market for a new saw and had the exact same consideration. I looked high and low and only found a small amount of info on 576xpAT milling. There's a couple videos out there showing people doing it, but I wanted to know about long term usage and wear, and there was little to be found. Mind you, I only intended to mill softwood less than 15" wide. This is what I determined:
The 576AT will certainly mill well on softwood less than 20" (it has quite high torque), but like with any milling saw one shouldn't overdo it. The biggest concern would be heat. As the saw automatically adjusts, running it "a bit richer" as suggested may not be possible. People say that one needs to slightly "de-tune the saw" when milling, and the additional fuel helps with cooling.
Also, guys tend to increase the fuel/oil mix to 40:1 and even 32:1 (and then, as mentioned, richen up the carb to compensate for this). Though 32:1 might be a little too much (BobL on here suggests staying at 40:1 or even 50:1, and he is clearly experienced in milling) I would wonder if the AT will have long term issues with 40:1. However, I've heard that someone once accidentally put bar oil in the gasoline tank of their 576AT and the autotune actually compensated for it and the saw ran! B ut generally, you may want to keep it between 40:1 and 50:1, and this is assuming that it is not a saw dedicated only to milling (if it is, go with a bigger saw).
Another heat factor is bar length. I intended to only run a 20" bar and this would probably have been fine. The longer the bar, the more power needed to pull it, and therefore the more heat created. I'd say that 24" and less would probably be OK. Over 24" would put a lot of strain on the saw in a mill, especially over the long-term, and you probably shouldn't be milling that wide of wood with 73cc anyways.
Of course there's also the chain, and like the bar, dull chain sucks more power and causes more heat. So keep it sharp and get ripping chain.
So I concluded that the 576AT would be fine for occasional milling of not too wide of wood (and not too hard of wood if over 12"), but to take it easy and let the saw cool well between cuts. Maybe even pause part of the way once or twice during cuts to also allow itself to cool. And its important to understand that letting the saw cool itself does not mean turning it off: just let it idle and allow the internal fan cool the engine, maybe for two minutes or so (turning it off will actually increase the temperature the first minute or two. This may cause overheating and seize the engine while off). And in that regard, be sure to keep the fuel tank full enough. Running out of gas halfway through a cut when the engine is hottest would therefore be bad.
Ultimately though I opted with a slightly bigger saw and its being delivered at the moment. I seriously considered the 576AT for a while, but due to lack of dealer support for me I opted with not getting an autotune saw. But I hope my input was of help. Good luck!