The shutter speed issue is hard - Canon cameras are somehow notorious for requiring faster shutter speeds to freeze action ...
I think the issue depends on the sensor/microlens layout ad the subject matter in big part. Simulations or better real tests are the only way to check.
That's interesting! I have no idea why the microlens layout might have any bearing on the shutter speed required to freeze action. Testing such issues using the cameras on a tripod should be no problem. Get your friend to drive by in your Ferrari at a constant speed whilst you photograph her.
Testing hand-held shots is more problematic, with or without image stabilisation, since camera shake varies from shot to shot.
The old 1/FL guideline for shutter speed applied to 8"x12" prints from the 35mm format, and that would apply equally to the D3 and D3X since both cameras are of the same format.
The impression I get is that, on a tripod, the D3X is no sharper at ISO 1600 and above than the D3, and the 5D2 is hardly sharper than the D700 at ISO 1600 and above.
If the subject is moving, then both cameras will benefit from an increased shutter speed at ISO 1600, to the same degree, and both images from each camera will still be equally sharp (or unsharp). I would predict the same would apply to camera shake, if camera shake were constant.