Supermarine Spitfire Mk. IX

3rd incarnation of Spitfire. With stock configuration it's a handicapped Spitfire Mk V - 10 km/h slower, with higher minimum speed and longer turn. The only difference is 100 meters better optimal altitude.
Fighting against tiers VIII.
Another problem is that usually retrained pilot, losing 10% of basic proficiency, makes sniping harder - Spitfires Mk. I and V were able to snipe enemy planes at the maximum distance. Fresh Spitfire Mk. IX can reliably deliver damage at half of maximum range.
As slow dogfighter, it suffers also from various bullet velocities - 20mm Hispano Mk. II have 378 of whatever, later Hispano Mk. V 444, and supporting .50 Brownings M2 333 - in turnfight, depending on cannons heat level, lead has to be shorter, to pump damage from cannons, or longer, to put machine guns to work.
Each developed module helps this plane to go from crap to mediocre, but even fully developed doesn't change the rule that UK planes at tier VII suck.