The 1/250 FP high speed option in E1 allows the flash sync to be greater than 1/250 if that is the normal shutter speed when using P or A modes. For example if I set my ISO to say ISO 400 and want a narrow dof so I set the aperture to f1.8. If the normal shutter speed under these conditions are above 1/250 then that is what the flash sync will be set to. It could be any shutter speed up to the top speed of the camera.
If you choose the standard 1/250 sync speed that is the top speed that you can use when in M or S mode.