IIRC, half duplex only means the communication can happen in one direction at one time, doesn't mean the speed will be cut in half. Most networking traffic is one direction. 8b/10b encoding applied in all real world ethernet traffic, not particular in this case.
Indeed, half duplex means that traffic can only send or receive in any specific time domain, and no, it does not mean that overall speed is cut in half, but it
does impact your theoretical speeds, which has a real world impact when you approach those limits. General traffic over ethernet is very much bi-directional, with ARP, DNS, TCP, and SSL/TLS traffic all requiring varying degrees of acknowledgements and responses. If you are multi-tasking, the situation gets worse, because you are limited to half duplex communication. If you have a large upload going while attempting to download, you can expect the time sharing needed to have an impact on speeds. None of this stuff matters if you're just casually using your link, but if you're for some reason trying to eek out the maximum performance of your link over a USB 2.0 connection, there will definitely be a performance impact.
Your second statement indicating that 8b/10b doesn't matter doesn't appear correct either, as it most certainly does.
If you have a 1Gb link, unless you're using fiber, it's not using 8b/10b anyways, but that's immaterial to the current discussion (because fiber based ethernet runs on the physical link at 1.25Gbps to account for this), but if you have 300Mbps of traffic, and 1Gbps of bandwidth, then it doesn't matter if you have 8b/10b encoding because you have enough bandwidth anyways.
But if you have 300Mbps of traffic, and you have a half-duplex link of 385Mbps because of 8b/10b encoding, then it most certainly can affect your traffic, because within the same time domain you would have half that download bandwidth available assuming in / out traffic took equal amounts of time.
According to Google's own analytics, for a certain amount of general web traffic, it takes about 11% of the data used to download to upload information to receive it. In other words, if you download 10KB of data, about 1.1KB of data is uploaded to receive it. This only works as a rough estimate, but again if you're multi-tasking, a half-duplex link can have a noticeable effect on traffic capacity over USB 2.0. Remember it doesn't have to be high bandwidth, it just has to take a long time. For instance, if you have 5Mbps of upload speed, and you upload 6.25MB of data, that would require the same time allotment as downloading a 375MB file on a 300Mbps download link. If both were occurring simultaneously, on a half-duplex link, without accounting for each operations respective traffic transmitted in the opposite direction, that 6MB upload would cut your effective download link speed in half to 150Mbps (because all things being equal, splitting up the time domain amongst both operations would require twice the time allocation).
My estimates of about 160-170Mbps appear in line with other people's real-world findings, so I don't see anything particularly wrong with my statements above.