On Development Preview of 4.5, seems like BMiner takes a lot longer to get up to top speed than what the benchmarks will allow time for (even precise).  To get the right speed for comparison, leave it running on a miner and save the hashrate.  So far, it is indeed faster than Excavator by a noticable margin.

4.25KH/s on a rig with Excavator
4.47KH/s on the same rig with BMiner

Ran for about 10 minutes before I saw the speed level out.