Any one has experience with tRFC timing on heavily binned b-die?

It's running 3900 15-17-17-34 CR2 on 1.55V. After a bit of research I found out tRFC should be able to do about 280 to 300 on b-die at that speed. Yet I am able to run it at 250 at 100% HCI coverage. Did anyone have a similar experience?