Snaphook has some good comments. I'll add a few of my own without repeating too much of what he's already stated.
Basics that helped me go from under 5% hits, to 10-15% without even really trying anymore-
1- Set your convergence close. I set mine at 275. (F4U)
2- Shoot planes at your convergence whenever possible. When the counter changes from 400 to 200, your at 299 yards. Aim and shoot, you'll be at 275 when you fire.
3- Aim small- miss small. Aim big- miss big. Pick a small target to hit, and even if you're off slightly, you'll still hit close enough to do the damage you want. Don't shoot at the plane, shoot at a part of it. Aim at the plane, and miss slightly, you've missed the plane. Aim at the cockpit and miss slightly, hit the wing, or the engine, or the tail, etc...
4- Shoot (practice) alot. Don't worry so much about actual hit % yet, that will improve on it's own. Worry about hitting your target. I don't subscribe to the "less is more" theory, where a limited ammo plane like a yak is recommended to "force" you into aiming. How do basketball players get better at free-throws? By shooting A LOT of baskets, or by trying real hard to score with just a few practice throws? Repetition is key. Shoot alot. Do that by taking a plane with a lot of ammo. I recommend a pony, or F4U, or Jug. Shoot alot, but don't just randomly spray. (Lots of QUALITY practice). AIM! Watch for results. Make corrections mentally, and try again. Lead needs to be "felt". It changes way too much to be able to describe where to aim. One hint though- if you miss due to incorrect lead, you'll almost always be missing behind him. It takes more lead than most realize, especially on those crossing shots.
You mention lead (too much? not enough?) Who knows?? You can't see the planes you're shooting at, so how can you judge lead? I believe that's what Snaphook means by being out of plane. In film one, at 5 seconds in, and 35 seconds in, you're shooting blind. In both instances, I'd normally set up at an angle, and shoot more like you do at 1:03 or so (where you missed the KI84). I want to see the plane and watch his path cross my gunsight. If I need to bury him behind the instrument panel I will, but I want to do that for as brief a time as possible. Shooting blind makes it impossible to judge lead, because you're guessing about when and where to shoot, instead of "knowing". It's randomly successful, but is based more on luck, less on skill, so can't really be improved on. Once you get a better feel for lead you'll be able to make those shots too, but you'll do it because you've already learned where to shoot.
Ideally, I would even go to a lag pursuit mode at about 57 seconds for at least a little bit, to set up more behind the KI, and not take such a high deflection shot. Even if you don't get the shot initially, you may be in a better position for a better shot a little later. Personally, I'd rather have one good shot opportunity than 5 "iffy" ones.
I didn't see the second film yet (slow dial-up).
MtnMan