Well, ships in Gothic just aren't all that flat. There are crenellations, buttresses, towers, folds, etc. Also, we are talking 40k weaponry here, burst cannons, lascannon, autocannon, multi-lasers, etc. That gives them a range of, like, a couple hundred yards! I really don't see BBs being more protected than even an escort, let alone a cruiser. Maybe if you're going for a lot of AA then the turrets/km2 could be upped to a rating of 3 I suppose.
I just don't see ratings of 4, 5 or 6. I suppose it's possible, if the ship is just bristling with AA guns. But if we're going to be using the whole "d6-turrets for attack runs" thing then I think that the most well defended anti-ordnance ship should be 4 turrets. Bonuses to hit or re-rolls to hit and/or being able to fire at both torps and bombers are possibilities, but give bombers a chance to overwhelm a target.