An A-Wing firing cannons isn't going to cause much damage on an ISD in a single run due to the chances of hitting more "empty" space than something solid; a crit there would be lucky to break down a system. An A-Wing crashing into a bridge is a whole different story.
LibrariaNPC's A-Wing comment has reminded me of things I hadn't thought of since I first discovered EotE, and I'm going to go ahead and change my mind on a few things here.
First, since silhouette 8+ seems to be the point we start measuring ships in terms of kilometers, I'd start treating them more like background terrain than targets for fighters. I'd even go so far as to say that Armor/Hull Thresholds are a moot point to fighters, as the most they can do to an ISD is inflict a targeted component critical or cause a major collision (like that A-Wing). Armor and Hull would really only come into play for the capital ships firing turbolasers.
So if my players were taking a squadron of fighters into battle against an ISD, their mission would be to take out gun batteries or any components that could be shot at from the outside. It would then be up to their allied capital ships to whittle through armor and deliver the coup de grace. This is a mechanic I would use solely against silhouette 8+ ships (5, 6, and 7 would still be fair game for fighter-launched torpedoes).
And back to the topic of point defense silhouettes, arguments made here have changed my mind a little. If the AoR book has anything on ground-based point defense weapons, I'd use that silhouette for the same weapon mounted on any ship. After all, you wouldn't use the silhouette of the planet it was attached to ... Would you?