Drilled holes are not accurate enough to conclude anything from testing their positions. At minimum, you should perform circular interpolation. Myself, I would perform a circular interpolation of about a 2" diameter hole in aluminum at a 20 ipm feedrate. Then set up a high accuracy indicator in the spindle and sweep the hole to see how much out of round it was.
You could run another test where you would machine a nice 'diamond' (ie., a square with its corners on the quadrant lines) and measure what size you get with a micrometer. Keep the feedrate moderate (like 20ipm) for one test, then bump it up to 100ipm and do the tests again. This should give you a feel for what the machine will do under typical conditions.