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Highpockets,
You can blame the Vokes sand filter for that. However, according to Air Wrench, she does fly pretty close to the numbers, though her climb rate is actually a bit high in all truth.
However, if you want to make her a bit more powerful, allow me to introduce you to airfile tweaking 101.
The easiest way to increase the performance of an aircraft in CFS2 is to adjust these two numbers...found in the aircraft.cfg file.
thrust_scalar=1.000
power_scalar=1.000
Increasing either of these numbers (to 1.100 or 1.200, for example) will give you a corresponding increase in performance. The higher you go, the bigger the boost. You can go the Bearcat route and make them into ballistic missiles with props, like he did with the original P-40 conversions, (just kidding, buddy) or you can just give them a small "shot in the arm."
A couple of words of caution with this though...first, be wary of adjusting the thrust scalar without knowing that you'll also be increasing torque, and you'll want to set the brakes before giving her the ol' windup. Second, remember that when you increase the power scalar, any increase means the engine will burn more fuel, shortening your range. However, over Malta, that shouldn't be much of a problem. Conversely, you'll have more ponies from the ol' Merlin to tangle with those pesky Axis fighters.