We've been as far back as B10 with EBCI. It isn't fun to watch family boarding going on while you wait to board even though you shelled out for EBCI but it can happen. On our last trip I was at the very end of the A group (A60) even though I paid for EBCI months before our trip.
Since we purchase Wanna Get Away fares, even with EBCI we board after the Business Select passengers (A1-A15), then those who paid $40 to upgrade to A1-A15, then the A-List members, then those who purchased Anytime fares plus EBCI and finally anyone who purchased a Wanna Get Away fare and paid for EBCI before I did. At times that can put you pretty far back in the boarding queue!