True, but on the same page the author writes that his engine problem was cured with a new engine. The original motor was sent back to Rolls Royce. He never said that his assigned Spitfire was any slower or faster than others in his squadron.
Yes but the engine was changed only after 3 months of efforts to get the original to work properly. And the a/c also had a drop tank jettisoning promlem which was never corrected. According to Neil on 2/3 of his cross Channel sorties EN237 drop tank didn't discard normally, it inpaled itself on the lugs, either sticking there and greatly reducing a/c's performance or whipping off sometimes later. But my main point was that according to Neil Mk XII was faster than either Typhoon IB or FW 190.