You can find lots of A-10 fanboys here: The A-10 Thunderbolt/Warthog: Air Force: "We don't want it" Army: We'll take it - Factual Questions - Straight Dope Message Board
What kinds of things were they doing wrong and what were their mistaken rationales?
Lemme just quote from reddit, from someone who seems confident with his numbers:
*The biggest difference is rate of climb and rate of fire. The A-10 can climb between 2 and 3 times as fast as the P-51 (6000 ft per minute vs ~2000 ft per minute). The GAU-8 fires at a simply incomprehensible speed, almost 10 times as rapidly as the .50 cals on the P-51. (450 rounds per second minute vs 4200 rounds per second minute). Also, the P-51 has 6 (sometimes 8) of the Brownings on board… but even combined the rate of fire isn’t close. Also, the A-10’s gun is good from a much, much longer range. The Browning has a listed effective range of 2,000 yards. But realistically the ideal air combat firing range was about a tenth of that… 200-300 yards. The GAU-8, meanwhile, has an effective range of between 1300 yards and 4000 yards. Against stationary targets like tanks. It might want to get closer when trying to hit a fast moving object like a combat aircraft. But it’s still going to be good from at least 2-3 times the range of the Mustang.
The P-51 has a slighly smaller turning radius (895 ft vs ~1300 ft) and a lower stall speed (95 mph vs 138 mph). So it’s slightly more maneuverable, but we’re not talking Zero vs Wildcat levels of difference.
And the A-10 doesn’t have to follow normal dogfighting rules. The P-51 (and most gun fighters) have to get behind their target and pour the lead on. This maximizes the rounds that hit the target, and gives a greater chance that at least one round will hit something critical and do significant damage. The A-10 can fire off a burst from anywhere they get an angle and still do considerable damage. The 30mm (at 960 m/s) is much faster than the 50 cal (820 m/s) leaving the muzzle, and it’s putting considerably more energy into the target with each hit.
Their top speeds are comparable at 25,000 ft if the P-51 is using WEP. Their service ceilings are also within comparison, at 42,000 ft for the Mustang and 45,000 ft for the Warthog.
All things considered, this fight is close enough that it comes down to the weapons. The P-51 can’t hurt the A-10. And the A-10’s main gun is designed to destroy tanks with just a couple of rounds.*
He’s saying the fight could be closer than, say, a Zero vs. a Wildcat in early 1942. Perhaps. Maybe he’s thinking too much of actual results between a P-51 and an ME-262.