So the guy responded again. Quite interesting stuff.
Okay, so we're talking about something called irreversible losses. This is when you heat up the magnet enough and cool it back down, but it won't retain its peak performance.
The magnet I work with is very similar to what they run in a Toyota Prius. But more specifically, this is the magnet used for torque in the motor. These are specialized magnets with extra dysprosium, a special element that is used specifically to increase the operating temperature, because these magnets are subjected to temperatures above 150o C. It also makes the magnet much more expensive, since dysprosium is rare. The neodymium magnet used to power the windows in the Toyota, however, don't get subjected to high temperatures so they leave out dysprosium. These magnets can't get above 80o C, or else irreversible damage occurs. To give you an idea of numbers for these window-motor-magnets, by the time you get to 100o C, you'll have a %2 permanent decrease in performance of your magnet. At 120o C, you'll have a ~5% decrease in performance. At 140o C, it's about 12% permanent decrease in performance. These magnets that have poor high-temp performance are the same magnets found in CD and DVD players, laptops, HDDs, etc.
That being said, I'm not sure what alloy your neodymium magnet is made of. As you can see, whether or not it has dysprosium plays a huge role in the irreversible loss of performance. In one magnet, it can be used safely at 150o C, in another magnet it can't get above 80o C while keeping its full potential. There are a bunch of different magnetic alloys that lie in between as well. If I got more information on your specific magnet that is used with your electric motor, I could give you more information.
But earlier you stated that you have a temperature gauge on your windings, and it's reading about 120o C. The really important quantity is the temperature of the magnet itself, and that won't be the same temperature as the windings. However, this is where my knowledge stops--- I'm not sure if the magnets typically run at a temperature above the windings, or below it.
What I take from this is that we need to find out what type of neo magnets they use to make these hubs! Until then, I think I can hang out in the 80c zone. With my air ventilated 9c I get it up to 120c pushing 4kw. With my new H series clyte in an oil bath I think I can do 4kw and stay at or under 80c.
edit: I just had one more thought: does the irreversible damage stack? As in at 100c you have 2% damage. If you take it to 100c again, do you get 2% MORE damage?