Well, there are ways to test a stator, but I don't see a max voltage. I will look at the manual more when I get home. From memory, I think it says each leg of the stator should be 30-40 VAC at 3-4000 RPM. There are three yellow wires that come from the stator. You would need to test 1 to 2, 1 to 3, and 2 to 3, if that makes sense. First ohm them all out with the quad off. Should be relative zero (.43 ohms). Then check each wire to ground, should be open (no continuity). Then check the voltage above.
I have not seen a bad stator yet, and from waht I have read, it seems most of the bad ones do NOT charge, and/or are physically damaged. Your charging system seems to be working correctly. It just seems like there is more of a draw on it (the charge system or battery) than it can handle at higher RPM. As a comparison, when my battery was going bad on my 2006, I got no battery light under normal use, but when I used the winch, the battery light came on and eventually killed the battery.
The polaris battery I had in my 2006 was a YAUSA brand battery and was a VERY good battery. It lasted 7 years and finally crapped out. You said you put an OEM battery in it. Is it actually a Polaris or YAUSA battery? I ask, because my Polaris dealer sold me an off brand battery when I went there to get a battery for my 2008 Sportsman 300. Brand was something like "bikemaster," and it was junk. Went bad once, I got it replaced under its warranty. It went bad again, I just junked it and paid for an AGM out of pocket. No problems since.
Is there a way you can test the quad with as much turned on as possible. IE lights, any accessories on, etc. In short, with more drawing on the battery/charge system, if the problem happens sooner (would indicate the system is not keeping up) or later/not at all (would indicate it is over charging).
Sean