Actually, the GAU is surprisingly ineffective against even 1970s-era tanks head-on. The pilots have always been told to go for the sides or rear because the attack profile makes it quite difficult to hit the (very thin) top of the tank. It may have a better attack angle than a mud-sucking armoured vehicle, but you are talking about frontal armour that can bounce a 120mm tungsten penetrator round. The GAU's a very impressive gun, but there's only so much a 30mm round can do. It's rated to 69mm of penetration at 500m.