Excellent!
But you might have to lower the warhead weight back down to normal, about 60 kg (130 lbs). Also, the Phoenix's performance at low altitude would probably not be nearly as good as its long-range intercept of high flying fighters and bombers.
A better choice would be something like the AGM-88 HARM, a non-ballistic air-to-surface missile.
(Interesting NASA article: "Phoenix Missile Hypersonic Testbed.")