Here's my numbers with the HH5b:
A is a Win7x64 computer with an 877.5 Mbps 802.11ac connection to the HH5b
N is a Qnap NAS with a full-duplex 1Gb Ethernet connection to the HH5b
B is a Win7x64 computer with a full-duplex 1Gb Ethernet connection to the HH5b and completely different hardware to A
From B to A: 19.9 MB/s Write, 17.6 MB/s Read
From A to B: 18.6 MB/s Write, 20.4 MB/s Read
From A to N: 14.3 MB/s Write, 20.3 MB/s Read
From B to N: 25.9 MB/s Write, 76.6 MB/s Read
(The NAS slowed from well over 40 MBps writes about three firmware versions ago, so I'm hoping they fix that soon.)
The NAS is on LAN3 and the Computer on LAN4, although as they are all going to the same Broadcom Ethernet switch I assume it doesn't matter.
I'm not trying the full set of tests on the HH5a right now as I need my Broadband speed to recover from the problems, so probably won't meddle with the HH5b for three weeks unless something goes wrong. The Read speed with the HH5a from A to N was clearly quite a bit better though. The NAS was on LAN3 on the HH5a as well.
(Edit) When I had computer A connected to the NAS via LAN it got the same speeds that B did here (after the firmware update that is).