The difference, as I see it, is that the early Fw 190 has a known gap between sustained performance and theoretical limits that leads me to emphasise the lowball numbers, but that's an attempt to obtain "realistic" figures for a squadron-service plane, not a wilful attempt to distort the results; I was setting them against the performance website's whole range of numbers for the Spitfire Mk. V with the Merlin 45 (which appear to be tested at its ordinary performance limits and weight, and as you say, always has a fairly unfinished airframe)...
(while trying to ignore ending the sentences with ... )
What was the gap, and who defined and proved the gap? And for what specific version of Fw 190?
List of Spitfire V serials that were tested at 350-360 mph, per the above listed RAE paper: W3134 (snowguard - installed in front of the ram air intake - really messed here the thigh up, both in speed and in rated altitude), AA878, AA937, EF644, EN946. Same paper notes that Mk.V was the biggest offender between the three mass-produced Spitfires of the day (Mk.I, V and IX) wrt. fit and finish.
It would've take a bit of nip and tuck, plus more effort into obtaining a better fit & finish for the Spitfire V to perform in the ballpark of Fw 190, unfortunately it was not meant to be, leaving for the Typhoon and installation of Merlin 61s on Spitfires (ironically, with better exhausts, carbs and BP glass installation) to more than equalize the game.
Wait, are you now arguing that the Merlin 46 numbers are understated?
Funnily enough, I'd definitely be prepared to consider the possibility that a Fw 190 has an edge over a Mk. V with a Merlin 46 at altitudes around 15,000-20,000ft; the question then becomes how many Merlin 46 aircraft were engaging the Fw 190, and if so, whether this was any sort of a good idea...