Phoenixes do +light, so on armored or normal units I think they do 5 x2 dmg. This low damage, high rate of fire attack is obviously horrible against armored units since such a large portion of the attack is negated by armor every time. I hadn't kept up with SC2 info at all until the beta came out, so I have no idea what the Overload ability did, but they have a Graviton Beam ability that's pretty good. It's a 50 energy channeled ability that lifts a ground unit into the air, rendering it helpless for 10 time units. The strength of that ability probably explains why Phoenixes are weak in general, since unlike Corsairs they don't do splash damage and as such are useless against large numbers of light air units such as Mutas.