I can't say I've tried it, but you're all but guaranteed to lose power running an 800 pipe on a 600. It's pretty complex, I can't claim to understand it all, but the two biggest variables are the overall length of the pipe, and the total volume of the pipe. The length should be similar between the two, but the 800 pipe would be "fatter." Maybe it's not as obvious on an AXYS, but I've got an old EDGE 800 VES pipe and it's easy to see the 800 is bigger in the middle compared side-by-side to the 600 VES. The problem with that is the excess volume dampens the pressure wave, you don't get a strong enough return pulse, and as a result part of the charge stagnates in the pipe rather than getting rammed back into the cylinder. A big bore motor might see some improvement – I'd definitely try it in that case. Thing is, any time you change cylinder size or port timing, ideally the pipe would change too. Not practical in most cases.
If I had money to burn, I'd definitely try the AAEN setup on my sled. It may be worth a little power, but the biggest gain is weight loss. The big downside, beside the price, is I don't see any chance of it being quiet. With regard to power, in many cases the stock Polaris 600 pipe was never beat by anything aftermarket. This is more specific to the 600 VES and HO, but I've read of people trying different pipes and either going back to stock or saying there was no difference. The weight is usually less, at least. The only big power gains I've heard of for my sled were twin pipes, but that means more tuning and more noise. Some day I'll probably get my hands on some "upgrade" pipes and give them a try, but for now I figure I'll save my money for a newer sled.