Regarding A-10 survival ability, it is manueverable and nimble, but it doesn't have speed. When avoiding flak, the speed drops some amount and the maneuverability goes also down.
Remeber that the current infrared guided AA missiles are can form an image of the aircraft on the sensor head. This means the missile could home in engines, but is more likely designed to hit the cockpit. I wouldn't be surprised if Russians already had such systems among their SAM launchers. Also bear in mind that A-10 can fire only a couple of Mavericks before the aircraft is inside the range of the close range AAA or SAM systems.
An aircraft that couldn't fulfil its mission objectives and was forced to turn back home is a victory for the air defenses, it doesn't need to be destroyed. In War Against Terror (is this the real campaign name?) the A-10 has surely been effective, there is no doubt about it as there is nothing that the enemy could have done to it. Take a look how well the Apaches fared in Iraq to get a view of the times when the enemy actually can do something.
I never really understood the need for Gatling gun, except the aerial combat and even there the effective rate of fire is about the same as the single barreled cannons. The videos, while impressive, are also misleading. Here the targets are clearly marked and the chopper is flying quite low. There is not much need for a high density of bullets if there is no visual of the enemy. And from the psychological aspect of having a gun that has huge rate of fire, me, I think I would be past caring if I got hit either by 3 or 30 bullets.
Well, I can give out that I'm from the old school where they said that the best density of bullets is one bullet/dead body, or, even better, one bullet/injured person. Don't need to carry that much ammo around then (Did anyone get the joke?). There is also another advantage on this, namely it is very difficult to tell where that single occasional bullet came from that took down you comrade. Personally I think that the huge rate of fire actually fixates in shooting a single target rather than killing or disabling the current target as effortlessly and effectively as possible. By the way this is coming from a person who needed three to five bullets with a machine gun to took down moving (by electronic means if anyone wondered) targets 500 m away. If you are a superpower and have good supply connections, it might be more sensible to differ from this line of thought.
However GAU-8 I'm not so sure of, was it really better to design a tank buster gun and then construct an airplane around it rather than lofting more Rockeyes in a single pass?