I'd say go all out and make the AT-ATs an actual threat to fighters, and give them the stats that an be dangerous and also tough to kill, and able to be killed by things other than bombs. Not to mention it gives the Y-wings something to worry about more than just time...which is kinda boring in some ways.
Especially sicne others did playtest those rules, I linked to you, worst case it doesnt work if you play test it before you run it for your Imperial Assault campaign and you go with the way you designed it (though I suggest play testing both to see which you like more).
As for the AT-Ats having 180 degrees for fire arc, hmmm, that is probably too much, sicne you never see the neck turn that much if I remember correctly. A slightly wider arc than the standard 90 could work if you wanted to extend it, but really it works well with the 90degree arc.