It's kind'a sad when building Hangers and turrets are "not worth it". I understand that with chokepoints, making defences too strong can lead to impassable stalemates, but as it is, I wish that when I spent that much time/money/resourses on a planet packed to the rim with defences, that it made an opponent flinch a bit. As it is, Repair bays seem the only thing worth really building; I wish it were otherwise (big space defence guns are my favorite part of most sci-fi movies/games). Heck, I'd love to see turrets get their range increased 200%, even if it reduced their damage by half. . .
Look at the pirate base... 19 or 20 turrets, and I've never lost a ship to those defenses. Kill the fleet, pick off the turrets one by one. Static defenses don't work against a mobile opponent. At best, they work to slow down an attacker or augment a defense fleet.
But your idea to extend the range of the turrets.. now thats a thought. Make them cover the entire gravity well and they'd be a decent defense weapon. Imagine jumping into a system with 20 turrets and all of them able to hit you. Maybe a sliding damage/range ratio, further away = less damage. It would make pirate base raids a little more dangerous.
But, I think SINS would loose a lot of it's appeal ( IMO ), if it could degenerate into a space version of trench warfare.