The one getting just 28 Mbps out of a HH5 something was clearly wrong, as if that was the case we'd see lots more posting about slow wi-fi on the Home Hub.
Though I do expect the HH6 to go faster, i.e. wifi more suited to the 300 Meg products (i.e. G.fast is probably going to be Openreach CPE for the initial year or two)
Not sure, 28Mbps is the exact speed my HH5 used to slowdown to (internal network transfers as well), on the older firmware. It doesn't do this anymore. This review is probably just picking up the same issue I was seeing.