The up-cant angle is to give a couple more degrees of lead in a turning fight. The problem with that in strafing is that the bullets sort of get lofted towards the target, and normal ballistic impact point is ahead of the plane that accuracy and weapons effect (bullet speed at impact) is severely degraded. As you dive down on a strafe pass in the F-15E, the pipper starts low in the hud, moves up, pauses for a second at the bullet apex well ABOVE the aircraft's flight path, and then slowly starts dropping back towards the velocity vector (the flight path indicator in the hud). With a computing air to ground gunsight, it is merely difficult. With only a fixed (adjustable but non-computing) reticle, the reticle is only accurate at exactly 2 ranges, once at very very close range with the bullets still lofting up over the flight path, and once at longer range when the bullets head back down. So you have to do a lot of computation on the ground, to determine the exact dive angle, speed, and firing altitude. So you have to have the pipper exactly on the target at the instant that the plane hits the exact dive angle and altitude, otherwise the bullets go... who knows where, either well short of the target or far far beyond the target.
The way the geometry works out, you have to dive at the ground in front of the target since the bullets are lofted up past where the plane is flying, and hope you judged things right so the pipper gets to the target before you hit your abort altitude or plow into the ground. It is possible to open fire at much longer range where the bullets are dropping back down, but the bullets are going so slow by then that they may not explode or cause much damage when they hit. I read a story of a guy who got hit by a 20mm training round... Some F-16s were strafing with a new, more aerodynamic type of 20mm round (PGU), but they didn't know that at low angles the new rounds would skip off the ground and travel for miles. So some rounds skipped well off the range and hit some guy driving in a car a few miles past the target. It lodged in his chest and he survived.
Come to think of it, that would be an interesting addition to the AH damage model... 20mm hits to the pilot at velocities below minimum for fuze operation could merely cause a pilot wound?
In a plane with the gun boresighted with the airframe, the bullets only drop, so the pipper is always below the velocity vector at tactically relevant speeds in a stable dive, so you are diving to a point past the target, not diving to a spot in front of the target.
