Removing warranty stickers to replace the TIM/pads or install a waterblock or aftermarket cooler is specifically allowed, and it'll just provoke a much more thorough testing and inspection of the card if returned.
So called 'liquid metal', or galistan (Gallium-Indium-Tin alloy) is very very nasty stuff, I never recommend it. It will pretty much instantly start forming an amalgam with any aluminum it touches, which is grey and crumbly and terrible for almost any use. It also "tunnels" through the aluminum, which can cause it to fail and turn to dirt-like amalgam rather far from the site of contact. All this for 1-2C drop? Versus things like zinc oxide which is so harmless and inert you can literally eat it without issue? (Don't do that. It's still not a great idea, bad for the digestion.)
I'll keep my 50 year old tube of zinc oxide "tractor engine paste" I got at a yard sale 20 years ago. It's as pure as any of the current brands, and it was a one *pound* tube for five bucks!