Many mistakes. The rectilinear Mustang wing gave more total lift than the larger elliptical Spitfire wing! The thinner Spit wing had a lower CL and more wash out that greatly reduced the total lift. But the Spit was more than a ton (2?) lighter and thus had a lower wing loading. If you look at the stall speeds at weights you get the idea. In addition, it was the wing twist under load near the tips combined with the wash out that gave the Spit the gentile stall and "Great" Handling early models were famous for. Later models acquired defects with the greater weight and larger props until the Mk-XIV was nearly un-usable for the first year of it's service. (Note the year long time between service entry and first kill!) Those "Snaking" problems persisted until the adoption of larger tails and were only completely fixed with the much larger "Spiteful" tail later on. But they built over a thousand Mk-XIVs because they were so desperate they had no choice. It was their only true 400 MPH plane. The Mk-IX does not count because of said defects which were unfixed! The Snaking in the Mk-XIV was so bad and major defect in a dog fight, but only a minor problem chasing a doodle bug. Watch A2A gunnery film to see the snaking and what it did to gunnery for your self. The Mustang had the exact same problem with the larger chord prop on the P-51H and had to add 8", or was it 18" to the top of the rudder and stretch the fuse more than a little to get a longer lever arm to fix it. All planes go through that type of thing, where mods are required to fix unk defects only found later in combat service. The P-38 was just about the only plane of that era that they found the defects before service, but because it was the only 400 MPH plane in existence at the time, they bought it any way! And do not be fooled by the published 414 MPH placard speed published in all the books. Do you think adding almost 700 HP would not change the actual, but un-tested speed? Just swapping props gave 450 MPH in one un-built but tested type.