Your bad indeed.
But honestly, I would still go with aftermarket cooling. The Asus is still going to sound like a lawnmower being passed through a woodchipper, while on fire and filled with angry badgers. That is simply the nature of stock cooling. "Fancy" stock cooling can be slightly better, but nowhere near enough to be tolerable.
Good aftermarket cooling will not only give you a much quieter card, it will also give you proper thermal headroom to overclock. Even with a overclock that will bring a 7950 to it's knees, my 7870 is still cooler at full load than any stock 78xx or 79xx is at idle. And it is still quieter than any stock cooling solution.
As for warranty.
I really could not care less about such silly things. Life is to short to limit yourself with arbitrary "rules".
But if you are into that sort of kink, you may like to know that some manufacturers do not care if you use aftermarket cooling. You usually just have to re-install the stock cooler in case of RMA.