Wing incidence on the Spitfire remained constant up until, and including to the Mk.XIX (and XX/a.k.a. Mk.IV). The wing design was virtually the same with the wing gaining slightly more weight. The washout, too, remained the same. Identical to K5054. The introduction of the IX pushed the CoG forward and the XIV did that moreso due to a steady increase in heavier wing structure, larger radiator/intercooler/oil cooler setup under the wings, armament and, most importantly, the bigger engines and propellers. The Mk.I had the CoG further back than any of the other marks.
As we saw, the Mk.I had fabric ailerons which ballooned at high speeds, reducing efficiency. The Mk.V was almost a Mk.I, and only had metal ailerons as a difference from a control surface point of view. The Mk.IX had its CoG placed more forward, and has a more powerful engine. The mK.VIII in the tests you chaps are looking at does have extended tips. Wing area of the said a/c has 40.1' wing span. The standard Spitfire wing span is 36' 10". In addition, the summary states that: "The smaller span ailerons combined with extended wing tip give the Spitfire VIII an inferior rate of roll."
At any rate, the less aileron there is the less roll there would likely be. Imagine putting the ailerons closer to the fuselage. The comparative lift/drop of one wing to the other will be very low and the plane won't roll effectively. Hence, the ailerons are near or at the wingtips. Chopping the outer part of the aileron is sort of like moving them closer to the fuselage by a small amount. The Mk.VIII, though similar to the Mk.IX in almost every respect, including CoG, rolls a little worse. The XIV rolls better (to the left) because of the torque. But was it tested rolling the other way? If anything, the CoG is probably the culprit for better roll of the Mk.XIV vs the VIII though they have the same ailerons.
EDIT: Try testing the planes flying at 400 mph and cutting their engines to eliminate torque. Or have I not paid attention.. has it been done already?