[...] Kahuna-style defence [...] is inefficient [...]
How could I improve my defenses?
Your defences as explained in your super cat thread are good for what they are designed for, that is, killing an enemy going for the command station, and I don't have any general improvements for that.
As I hope I made clear in the post you are quoting from, it was their use in the specific circumstances that was inefficient. There are different threats in the game, and while killing waves going for command stations is one of the biggest, it is not always the biggest one.
Specially, such a defence is not necessarily the best solution to the big threat Zincat faced - namely that of inflicting attrition casualties on an enemy that is passing through the system and too strong for him to stop (though in this case it can still be a good defence depending on where in the system the station is located), and it becomes an outright bad solution if that defence is set up the Zincat did, putting the command station directly on the wormhole that an exowave passing through would have to exit, thus guaranteeing that the exowave (which he was too weak to stop on the border) would smash all of his defences (including spire city buildings) before proceeding onwards, resulting in huge repair bills and huge holes in his defences until repaired for very little gain.
It was his terribly inefficient use of your defences, that I remarked on, as it violated all classical ideas of how to deal with an enemy of superior strength and, unsurprisingly as AI war is a pretty good strategy game, had the classical outcome when a weaker defender tries to stop the stronger invader in a straight-up fight rather that taking advantage of strategies of terrain, strong interior lines, and attrition.