IMO the spit IX is the most dangerous plane if flown well in the game. It can turn better than all but the spit V, can out-accelerate nearly all the planes, and is very stable as a BnZ platform if you choose to fly that way. The only major advantage most planes have against it is top speed, and unless you have E to make up for inferior acceleration, that won't even help you. (Not only that, but the spit seems to have an amazing ho ability, and paired with it's turning ability, can almost always force a ho.)
When I meet spits in the f4u I always try to get them to try to climb to me, then I will zoom climb away from them. All that's left to do is zoom back down on them and use the only real advantage the f4 has, roll rate. In a dive, the f4u can dive to a faster speed and match any turns the spit tries to make. The spit's only real weakness is its roll rate, and in some situations you can use it to your advantage.
Unfortunately I don't really know the best method for fighting spits in a 109. I haven't had much success, other than just keeping an altitude advantage and waiting until they expose their 6 to you. I'm sure Hristo or Hblair can tell you how to do it though.
bloom25
THUNDERBIRDS