Sounds to me like the guy sort of understood stuff, but not enough.
1/ Range extenders are generally bad news because the halve the speed due to relay functions. Also they halve the speed they can get not necessarily the speed if they were sitting next to the Hub.
2/ If speed dropped with Ethernet cable, then there might be a utilisation issue in your area. The VM bods, when they get to your thread in a few days will be able to say if that's the case.
3/ I am in no doubt as to what should be done if there is a wireless-through-wall issue in your house (which is likely). That would be a Powerline solution. The base unit would be Ethernet only (from the Hub); the other side of your home would use a WiFi Powerline Adapter (with an Ethernet poet available on it if you wanted to use that). In my experience, maximum you'll get out of that, all other things functioning well, would be 90 meg downstream.
4/ Regarding priority, the Hub does not allow you to set priorities for services you are using. A third paerty router would allow that. So your PS4 gaming would be contending with streaming and it might tempt you into thinkng that streaming has priority. If you bought a good spec third party router, you could prioritise the PS4 in the upstream direction (what you send) but you would have kless control over what is coming down from the Internet.