Wirbs and Osties will tear hell out of any IL2 dumb enough to venture within about 1.5k. That said, a suicide JABO plane is nearly impossible to stop with anything short of a 5" gun (even then, its not a sure thing).
+1.
For nay sayers, bear in mind that it would be much less effective than the 5" guns on the CV.
1) much smaller shell, which means it would need to hit very close to you
2) no proxy fuzes. the gunner will have to set the range for his fuze (which will be difficult, since range is not constant, and so he will have to guess your distance at the point of intercept)
3) no armor. It will be as easy to destroy as an auto gun.