Andrew, did you use the set up wizard in PHD2 for the SSAG?
Assuming you set up the guider correctly (FL of the guidescope, pixel size of the new cam, calculated the steps required etc), you should get similar readings.
The other point to remember is to use "On camera" under mount setup.
http://openphdguiding.org/man/Basic_use.htm
What was the RMS error on the SSAG compared with the DSI II? That should tell you a better story.
Hope that helps.
Bo