The simple fact on the Spit vs F4U thing (like other dissimilar plane match-ups) is that in order for the spit to prevail he needs to get the faster F4U slow and "pinned down" and minimize his options. If he (the spit) doesn't, he can't expect to win against it on a regular basis.
The simple fact of the matter is that if the Hog has speed and/or alt he can dictate the terms of the fight, and should be expected to do so in his favor. As a hog pilot I honestly look at Spits, Hurri's, Zeke's, and N1K's as just another scalp to hang on my lodgepole. If they're lower/slower than me, I can kill them at will. They may squirm around a bit and make things interesting, but unless they get me to make a mistake and blow my E to where they can keep the fight very "tight" they're in trouble, and I'm not. If I have 4-5K alt above ground they really can't do much to me unless I let them (I make a mistake). A good spit (or insert other plane) pilot can be a handfull for sure, but again, can't do much if the F4U doesn't play by his "rules".
If they come in higher they're a threat (but only because they have enough speed to get/stay close to me), but really only until I can fool them into putting themselves in a bad position. Then they'll normally revert back to a tight turn to block my shot, and if I fly E-conservatively for a few seconds I'm back in charge. That's the beauty of the faster plane types. If I'm faster, I can dictate the fight, and if I have enough alt I can scrub E/speed at will, briefly, without too much danger because I can simply go nose-down for speed again if I want/need to. Again, the choices are mine because my plane choice allows me to dictate the fight.
As Saxman mentioned, a "smart" hog pilot won't allow the spit pilot to dictate the fight, or to fool him into getting low and slow and "pinned down". As BaldEagl mentioned, the spit can prevail over the hog if he flies "smarter" than the hog and convinces the hog pilot to screw up, and then is able to quickly capitolize on that mistake before the hog pilot can take control again. The closer to the ground and the slower the F4U is, the better for the spit (notice I said slow F4U, not slow Spit).
So, that basically means that for the spit to prevail he needs to get the F4U to fight on the Spits terms. For the spit to control the fight he needs to maintain enough E to keep enough speed to make repeated slashing attacks on the Hog, and keep the Hog dodging, blowing his energy, and getting trapped. Can the spit slow down and "saddle-up" on the hog's six? Sure, but he'd better end the fight quickly, or the hog can take it over again. If the hog is allowed to sneak out and extend to get speed back the Spit is now pinned down and unable to get away. Saddling up is fine if you can end it quickly, but dangerous if you can't. Basically, the spit needs to switch gears to an E-fighting mode, where we normally see them used in a TnB mode of fighting.
Unless the spit has enough E/speed to control the hog, he's not dictating the fight. The same can be said for spits (or other "slow" planes) fighting ponies, 190's, 109's (or any other "faster" planes).
Or for that matter, my wife's cat vs my dog (JR Terrier)(He has abnormally fast speed for a JR). The cat lives outside, the dog lives inside. The dog RULES the ENTIRE YARD! But the cat RULES the PORCH. If the cat strays out into the yard, the dog chases her without mercy, and eventually she goes up into a tree until he goes away (long time, hehe, he's crazy about chasin'!).
But here's the kicker. The cat rules the porch, which the dog uses to enter and exit the house. On the porch, the cat can back into any number of places, and is awful formidable from the front end with her teeth and claws. She's also able to launch attacks, since she has safety so close behind her. She'll chase the dog off the porch, and set up on the steps and WON"T ALLOW him to come back up. She has food/water on the porch, and has no reason to leave. He sits out in the yard and whines until we come rescue him and escort him back inside!
See the correllation? Both have strengths and weaknesses, can dictate the fight under certain circumstances, and both are vulnerable in other situations. Recognize how/where you can dictate the fight, and use that to your advantage...