Durability-wise, I'm sure mammoth ivory is just as tough as anything else, it's been around for tens of thousands of years. However, when you order, make sure that the store has a good return policy...even better, ask if they can inspect the knife you're looking at. I had a great experience with KSF during ordering/returning.
I ordered a Mammoth Ivory Northwoods Hawthorne and it was a gorgeous knife, but at the price point, I couldn't live with the gaps by both bolsters and the fact that the scales weren't flush to the springs. I ended up returning the knife. The blue camel bone Hawthorne that was sent as replacement was half the price and had perfect fit-and-finish. It seems that the price hike is for the mammoth ivory scales alone and it doesn't necessarily mean you're paying for better fit and finish.