I have to say that the double pixel 45 degree lines and generally fattened up outline stuff does not look great to me in most cases. It works better in some, like the back of the head of Viewtiful Joe, than others, such as long 45 degree lines, imo.
Also the fact that a line like this should be lighter than the colour it is intended to be, due to rules of antialias and if not done right a diagonal fattened line will look significantly thicker than a straight line).
I like the general idea of keeping clusters efficient, but I do not see the problem with single pixel outlines at all (look at Mia's lovely work, among others), or with single pixel step AA. If you have the colours to fit in 3 pixels of different value for AA in a place which ideally should have 3 pixels worth of AA, then 1 pixel, 2 pixels, 4 pixels, or 3 pixels consisting of less colours will likely give a worse result.
I have not had the time to experiment with this more, so this is all just from analytical thought and not from proper testing, which I will get to once I am back home (mid Jan) and have some time for it.