It's an example of diminishing returns. Going from the cheapest cog available up to, say, a $20 All City one, there will certainly be an improvement, but it might be only 10% better in terms of real, noticeable benefits. To then go from $20 up to a Phil Wood or an EAI Gold Medal (like 100 bucks probably?), there might be only an additional 2-3% gain. How much benefit the Dura Ace model can offer is subject to debate. Obviously, the DA is better than your stock one, almost without a doubt... but it's probably not a lot better. Perhaps only a tiny bit, or perhaps enough to be noticeable while you ride.

Do you like the Dura Ace? Will it make you happy to see it on your bike? Can you afford it? If so, use it. If you'll only ride that bike twice a year, your kids are going without shoes, or you have to skip lunches for a week to afford the cog, send that silly thing back for a refund.