I suggest rendering with higher anti-alias settins, because floating-point math used in color calculations results in some roundings errors and AA might help to get rid of them.
I used 
+w1600 +h1200 +a.01 +am2 +r3 +j.5
and then resized to 800x600, it renders in about 15-30 minutes on 2GHz, tiny memory usage.