SRG(Signal Research Group) did this study with the support of Accuver Americas (XCAL, XCAP and XCAL-Solo). 

They did some preliminary testing of MU-MIMO, based on testing that we did in the Verizon Band n77 network in the western suburbs of Minneapolis.  Although the contents of this report do not reflect a full-fledged benchmark study, they do show that MU-MIMO is alive and well, and potentially heading to a 5G network near you soon.