I think it is a truly bad bellybutton weapon, and it puzzled me as well why this wasn't used more.
Because by that time of the war, most of the major airforces pretty much concluded that the most efficient way to bring down a plane was to use explosive rounds to completely tear away the whole surface of the plane which holds it together - rather than hope for a clandestine kinetic penetration to damage a vital, inner part.
Such differences between AP rounds and HE rounds are not adequately portrayed in AH, but in FB/AEP I would absolutley prefer the F-4 over the F-2. The MG151/15 has a pretty good chance of cutting control links, puncturing fuel tanks, and wounding the pilot.. but in most cases, while severely damaged, the enemy plane can still hold together for a considerable amount of time unless something really important is hit(like causing fires, killing the engine, killing the pilot or etc etc..). Also, unless a continuous volley of AP rounds connect at an important spot like the spars or supports, it is very hard to snap wings off with a MG151/15.
On the contrary, while a bit poorer in ballistics, the MG151/20 mounted on the F-4 really shows a difference - the HE rounds are much more efficient in overall effect compared to the 15mm AP rounds.