Just remember that XFX and Asus put stickers on the screws meaning you can't even properly clean your heatsink and repaste without voiding said warranty. Since you're so dead set on warranty I suggest buying a different manufacturer besides the above two and simply buy new.
I thought XFX was fine with modding/overclocking/disassembly? They were with nVidia, at least. Did that change when they switched to AMD?
I've already given my educated guess since I've dealt with Gigabyte a lot in the past. Just assume the answer is no. Even if they did, the turn around for an RMA from them can take up to 6 weeks or sometimes they can even lose your damn part. I make it a point to stray away from GB.
Never had an issue with them when I sent my motherboard in for RMA. Was fairly quick turnaround time as well. This was back on a 775 board, though, so it's possible they've changed.