On a 5 minute sub working at 1500mm focal length on 5.4u pixels I'm getting a FWHM of 4.01 pixels, that's 3 arc seconds FWHM. The focussing image (1 second long) was giving 3.3 pixels FWHM, so that suggests the guiding should have been about 1.7arc seconds FWHM or 0.77arc seconds RMS (PHD agrees). I'll have to check the log and see if PEC might be able to make a difference now.
Not perfect, but certainly a good big step in the right direction, and leaves me with bigger fish to fry, like getting a coma corrector and filter wheel installed.
Time for a little happy dance!