"People have been whining about Spitfires since the dawn of online flight sims. They just can't accept the fact that Supermarine built a better dogfighter than anything the Yanks or Krauts could conceive. "
The problem, at least in all the flight sims I've ever been a part of, is that sims tend to model the good points of many of the most famous fighters (Spitfire, P51, 190, 109) but not so many of the bad points.
For example, the Spitfire, while unsurpassed as a close in scrapper, was unable to compete in a high speed energy fight with such as the 190A. It exhibited poor maneuverability at speeds in excess of 300 mph, was slow to accelerate, had a poor dive and zoom climb, and didn't have the top speed (except for the Mk XIV) to engage/disengage at will.
The "less able" Yank fighters, on the other hand, generally maneuvered quite nicely at high speed (the P51 and P38J-L both easily outrolled the 190 at top speeds), posessed dive (and level, in the case of the P38) acceleration and zoom climbs well surpassing that of the Spitfire, and were generally fast enough to close on, or get away from, a bandit.
In these flight sims (most of them anyway), the high speed maneuverability of the Spitfire (as well as the 109) is generally much better than in real life, and since it's somewhat difficult to model dive acceleration and zoom climb independant of horsepower/weight and rate of climb, we see Spitfires that outzoom/outdive a P38L. Unfortunately, since these shortcomings in real life aren't found in the sims, these real-life "tough customer" fighters become nearly unbeatable, and matchups that would've generally turned out in favor of the faster aircraft in real life end up going the other way in a sim.
Hope this helps.