I have an STF8300M with 8 position filter wheel and the EOS adapter plate. It focuses perfectly behind the EOS 70-200 f2.8 lens. If I add the OAG to the camera, moving it back about 18mm, will that still focus behind an EOS Lens?
Probably not. You should not need to guide when using a camera lens; any remotely half-decent mounting should be able to track accurately enough. So if you want an OAG for other reasons, just remove it when using the camera lens.