Excerpt from Fighter Aircraft by Alfred Price, who's citing a summary of comparison of the P-51B against the Spitfire IX conducted during WWII in Britain and the States.
"Rates of roll: Although the alierons feel light, the Mustang cannot roll as quickly as the Spitfire at normal speeds. The alierons stiffen up only slightly at high speeds and the rates of roll become the same at about 400 mph."
Since I'm no expert on the P-51, it could be that there was an improvement of with the P-51D, but given that the Spitfire XIV was said to have an even more significant advantage over P-51(B), I'm not really sure.
To be honest, I have the impression that TK assigned the Spit IXE with clipped wings to the IDF simply to make it easier to distinguish them from the Egyptian Spitfires, not to mention that not all Spitfire IXE had clipped wings. That was only the case with the Mk XII.
Btw, I have accepted a long time ago that the AI pilots simply don't care what they are flying or what they are going up against. Unlike a human pilot they simply can't know the "DO and DON'Ts" of the aircraft they are flying at that point.