For spits, i find that if you keep the fight very fast and use the excellent instantaneous turn performance of the mustang to be very aggressive on the merge, you will find you can be at least the equal of the spit for the first couple of turns. If you have alt and keep the fight descending (to keep your speed up), you can give any spit a run for its money until you run out of alt. The last thing you want is to run out of speed and get into a slow sustained turn contest with it, cause you will never win that match up. Keeping your speed up and being aggressive can yield results. If you haven't gained the angles you need within the time window you have your speed up, then you can always withdraw, use your speed advantage to gain excess E, and come back for seconds.
La's pose a bigger threat than spits. Spits are entrees. Las are the main course. I always go after the Las first. Keep the fight between 8 and 12k for the D pony, and above 12k for the B pony, and you can be the equal of any La, espc with the combat flaps. Dont get below that unless you have a good positional advantage, otherwise it will turn in the Las favour. Above 8K, you can still do "tactical withdrawals", below that you cant.