Although I am not a KOTH event vet (as they are usually held at times I can't attend) we've done a lot of squad practice with alt caps in custom arena's with a violent wind layer at the cap alt and I've fought in Monday Madness which has an alt cap that a lot of people fly at or near, so the tactics are similar.
A few concepts that may be important for you to explore/understand, if you don't already, are one-circle vs. two-circle fights, turn radius vs. turn rate and corner speed vs. sustained turn speed.
That may sound very technical, but it's actually kind of simple in application. The initial decision (at or before the merge) will be: do you want to fight one-circle or two-circle? This can be based on recognizing a speed difference between you and your opponent (or a matter of preference based on what you are good at).
If you know going into the merge whether you are faster or slower than your opponent (based on SA), or if you can suss that out during the first merge, that can help you decide what type of turn (or merge) to push for. Generally speaking, if you are carrying more speed than your enemy, then pushing for a two-circle fight (turning nose-to-tail) will be better (which is kind of what Junky described in his flat turn with speed scenario above). If you have less speed than your enemy, then going one-circle (nose-to-nose turn or scissors fight) will be better, because you will be able to turn inside their turn radius (as Violator describes in his suggestions above).
In other words, if you think you are slower, or are in a position to get slower more quickly than your opponent, go scissors fight and turn inside them. If you are faster and can maintain that energy advantage, you might be better turning nose to tail and using turn rate (instead of turn radius) to win position. Knowing both ways of turn fighting is important because a fight can transition back and forth, such as a rolling-scissors (two circle) fight becoming a defensive scissors (one circle).
Knowing the "speed stuff" (corner speed and sustained turn) can be helpful as well, as it will let you know when to use what vertical space you have to control your speed. For example, if you know you are above corner speed, you can turn slightly nose up (assuming starting at 5K in the scenario you described) to scrub some speed and get closer to corner speed which will get you to max turn rate. If you are slower than you want to be, you can make a nose down turn (or low yo-yo) to gain speed in the turn (which was also described above). Knowing the best sustained turn configuration (flaps setting) and speed for a given aircraft can help if you do wind up in a protracted nose-to-tail chase (lufbery) fight.
Angle fights aren't always about out-scissoring your opponent, but they do often wind up there, especially once you've gained position and your opponent goes guns defensive. So, some scissors practice is always good. But if you've gotten to that point, you've won the merge part of the fight!
Hope this helps some too.
By they way, posting some AH films of your merges and fights in the films section might be a good way to get feedback as well.
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