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Supposedly they will honour warranty as long as there is no damage to the pcb. There's a post on hardocp from nvidia customer care stating this, though whether that's etched in stone or not is another issue. Its not unusual to get different agents saying different things.
https://hardforum.com/threads/nvidia-watercooling-warranty.1969526/#post-1043880671
I thought with the founders edition cards you had to removed the front bit of facia that is glued on to remove the cooler, so any warranty claim will show obvious damage if refitted and returned.
Aye there seems to be some disagreement with what is covered. I have found a few places saying it "should be" and others suggesting that it would not be supported. Whilst others have told me that have to accept it due to some law as long as it is not damaged.
Be nice to have confirmation.
But unless you physically damage the card there is no way to tell it has had a block on it unless you round off half of the screws it will look as factory standard.
You could possibly tell via thermal paste. But if in doubt just fit the same spec paste before returning the card.