With the drag that the pony had on its wings, why is it that it can't stay in a turn w/ flaps with spixteen?
Maybe in "THRASH99 World" the P-51D (or any other Mustang) could out turn a Spitfire Mk XVI (or any other Spitfire) but in reality, even with flaps the Mustang could not out turn or turn with a Spitfire Mk XVI. Hell, the Mustang couldn't out turn a P-38, why would you think it could against a Spitfire?
I would really like to see what sources you have that claim the Mustang could, if anything it would be a very funny read.
The spixteen out corners it like nothing.
See my above comment
Why is that spixteen out accelerates the pony even though they have the same engine?
The Spitfire Mk XVI was a lighter plane, so I believe it has a higher power to weight ratio than the Mustang did and the acceleration bit also depends on the altitude both planes are at. The Spitfire Mk XVI was primarily designed as a medium to low altitude fighter and its engine is tuned as such. The Mustang on the other hand was a high altitude fighter and it's best performance figures reflect that. At altitudes where the Mustang was designed to fly at, you will not see the Spitfire Mk XVI out accelerate or out climb the Mustang.
Last but not least, why is it that the spixteen out climbs the pony like nothing too?
See the above answer.
Even due to the less weight that the spixteen has, talk to any historians about the planes and they'll tell you that the pony was way better than spitfires,
Any historian that will go on record and say the Mustang was far superior to the Spitfire or vice versa is not a historian worth a damn.
One famous USAAF fighter ace said of the only difference between the Mustang and the Spitfire was that the Mustang could do them over Berlin whereas the Spitfire couldn't.
talk to any pilot who flew spitfires and 51s during the war and they will tell that pony was a dominate fighter over the spits. So my question is for this, why are the later spitfires, 14 and 16, better than the pony? 
You should do yourself a favor and read the history of the USAAF 4th Fighter Group and of the RAF "Eagle Squadrons" and then come back and post your claim of former US Spitfire pilots saying that. A lot of hearts were broken in the 4th when those that flew Spitfires had them taken away.
ack-ack