That's a horse kg 

That's a absolute cracking KG.... don't see many above 60cm, and to get it on a jigg is sick. I have played around with 40lbs black magic leaders, especially when using the smaller jigs. But when dropping deeper i switch to 60lbs just mono no fluro for me.

 Sounds like your all over it Sea Goat. I use 40lb most of the time and occasionally go up to 50lb. Like any lure fishing, lighter the better. Less resistance in the water. Most important thing is matching your rod to the jig weight. This will make the jig "dance" a lot better. Test the jig just under the boat in view with different actions to see which way suits the jig. Each type of jig needs a different action to make it do it's thing. Keep persisting with different techniques and jig colours and it will happen. I will warn you though, once you crack it, it is very addictive. Hope this helps. 

When jigging I've used light mainline/light leader but now prefer 100lb tasline with 80lb sunline flurocarbon leader (I'm not fussed what brand flurocarbon, but that's just what I have ... but a long leader is important, min 1.5m, preferably 2m+).
Why would I go so heavy?
Because when I jig along side others who go light, I seem to do just aswell, and I don't lose as many big fish.
I just find that works for me when jigging for Dhuies.
For Dhuies I like to be jigging straight up and down, so I do a fair bit of retrieve and cast ahead of the drift or in smaller boats, preferably reversed up to the drop zone. And because I'm doing mostly straight up/down jigging, I don't benefit when using thinner line.
At the end of day, try lots of setups and you'll find what works for you.

And having said all that, I ain't never caught a thumping King George jigging before (ie. there's compromises and good/bad in every setup).

 I use 40lb leader for my artificials, but if I was to hook a monster I would probably regret it. What I would say is throw a split ring onto the top of that jig with a swivel attached and tie off to the swivel. It will free the jig up a little more to work easier. I was using my jigs the same way and actually  noticed a difference in bite rate when I added the swivel. If you add the split for the hooks to the swivel also you can just swap out the jigs all day with out re tying knots also.

 Thanks guys! Good tips. Will definitely whack a split ring and swivel on from now on! Sounds like a much easier way to change it up.