A GAU8 will destroy any armored vehicle today. Tanks aren't armored equally on all aspects, and the top of the tank (where the majority of the GAU's rounds will impact is usually the thinnest. Now the A10 won't survive long enough against a decent opponent to employ the GAU, but that doesn't matter since either a Maverick or a SDB would be the preferred weapon against a tank. The GAU is really intended for soft targets, not tanks.