I'm not sure, but I think that extremely shallow tapers turned on a cnc are still 'digital' in that the X axis moves a bit, pauses while so and so much Z travel occurs, then another X pulse. There is simply no interpolated motion possible when the command is less than a tenth, or whatever your machine resolution is.
Apart from that, have you ruled out relevelling the machine to get a straight cut? .0004 in 8 inches is straight, AFAIC , but a little tweak on the levelling jack on one side of the tailstock end should dial it in straight.
I hope you were turning with coolant all over the part and the toolholder. Only .0002" is the real error!
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)