I'd much rather delid the CPU than to change out the thermal paste on the Asus Strix card. I think cards in that league and as what I have read in reviews, are already using pretty good thermal paste. I think past the warranty period is a better time to change out the thermal paste and not worth voiding the warranty for such an expensive card. AFAIK, only the ZOTAC and Gigabyte cards locally can be removed. Gigabyte distro sticker is on the screw but that screw is removed from the inside of the card so you will never strip the sticker in anyway. I know cause I had one before but sold it for the MSI Gaming X before I wanted to apply some Kryonaut to that. The Gigabyte cards are worth doing it cause from my experience and from other users on OCN3D, Gigabyte did a bad job on pasting the GPU.
So I think delidding your CPU will provide more gains than changing out the GPU thermal paste. If you wanna cool the Strix further, you can probably put a 120/140mm static pressure fan blowing over the backplate cause that helps temps by 3-5 degrees depending on your temps. I do that and it helps.