I think there is no specific reason to put the QPD in the focal plane of the lens. I don't think that the SR2-shift will be so strong so that it becomes necessary to suppress this movement (however, I can be wrong on this point, so please correct me).

Also, the reason why I suggested to use a lens was to decrease the sensibility (temporarily) so that VIS can do some alignment measurements of the suspension itself. In order to further decrease it, try to put the QPD more further away toward the image plane of the lens!
(of course not exactly in the image plane^^)