I have run in the Kinvara 4, 7, and 8. A great shoe got better every time in my opinion. The latest versions definitely feel more plush, but I think they are as fast or faster as the older ones.
Anyway, my two cents on the questions in this thread:
Marathon shoe? The 8 was a fantastic marathon shoe, I would expect the same from the 9.
Mileage? Very individual and also depends on shoe rotation. My 7s were dead after about 450 miles and I couldn't bring them back to life (tried 2-3 times). The 8s felt similar around 300 but are staying strong a bit over 500 now. You never know and can't put an exact number on it. You've just got to feel when shoes are done (hint: When you don't feel it, they're still good.).