Code:
...sb600 for light. The f6.3 shot was at 1/500th the 4.5 at 1000th.
At those shutter speeds you are forcing the flash to do multiple flashes for each frame because the entire frame is never uncovered at one time (flash sync is 1/250 or slower). The various exposures may overlap, and may not line up perfectly unless you have the camera bolted to a concrete pier. To do a fair test I suggest:
1. Do it in bright light on a tripod, using a shutter speed faster than 1/500.
2. Do not use JPEG. Set the camera for TIFF, so that you don't confuse compression artifacts with optical problems. (JPEG is not at it's best with text on white background.)
No semi-pro camera will produce wonderful results out of the box. You have to learn your camera, fully understand what is going on under different shooting situations, and learn to use it. It is a tool, it is not a magic wand.