I had the EM5-II and it's a smashing camera, however good at tracking it isn't and this is a case where equipment does make a difference. Add to that you were using the 75-300mm which has poor AF, especially at the tele end and I'm not surprised your results were poor TBH
I also have the D750 and the difference between this and the EM5-II for tracking is night and day. Folk are saying the XT2 is as good as high end DSLRs i.e. D750's and 5D3/4's so Id imagine you'd find the XT2 a lot better than the EM5-II for BIF. Obviously I'd recommend trying one out for yourself first