I wish we could the KitKat update now, but to be honest, 4.1.2 performs better than 4.3 on the S3.
As has been said, 4.3 is a lot heavier than 4.1 but KitKat would be nice and quick again as it offers performance improvements making older hardware smoother again which would fix that.
I would recommend to anyone hanging for 4.4 KitKat to just go and try out a custom AOSP based ROM like SlimKat and see how good KitKat is for yourself on our device. It is not hard to do and if you make a nandroid backup, you can always go back.
If you are desperate for the Samsung interface, you could always get a theme with your launcher of preference
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