The P7 reflector would be too big of an hole for the SST-50, if you order the reflector for the Cree XR-E then that would be a perfect fit.
Actually my concern is about the beam. Any idea about the beam? I was thinking the P7 multi-die reflector should work okay for the SST-50 but not sure if an XR-E reflector would work better than a reflector designed for the P7.