In the game, does the dive brake on the L model do much good? I deployed the brake and chopped the throttle, put the nose down and the airspeed still built up rapidly. Things started shaking so I slowly pulled out of the dive.
Not what I expected.
Because it is not a brake, it is a flap. It isn't meant to slow the plane down at all. It's meant to keep it from becoming impossible to pull out, really, for pilots who forced the compression issue. It has never been called a dive brake by anyone who knew what they were talking about.
A P-38, without dive flaps, flown properly, will recover from a steep dive, as soon as it reaches thick enough air. In AH II, this may not always be true, but it should be. In real life, according to all of the real P-38 pilots I ever spoke with, the only way to auger a P-38 in a steep dive was to either keep forcing it down past the point of no return, or panic and stop flying the plane. It simply can't get going so fast that it won't recover, if you use the correct method, which was to keep power applied, you'll need it, and keep a steady pull on the yoke until you got down below 15K or so, and the plane would recover with room to spare.
Also, Widewing recently posted that the dive flaps do not work if you use auto trim, which you should not use in a P-38 to begin with.
If you're looking to use a P-38L as a dump truck to deliver ordnance in a screaming vertical dive, just choose another plane.
