Speed <> Bandwidth.
Most tests and certainly marketing documents list performance of SSDs in terms of bandwidth achieved in uncommon/extreme usage scenarios.
The speed the uncited Intel representative most likely referred to is latency - in this case how quickly a drive will return an answer to a request for data.
AFAIK, all Optane versions of a given model achieve the same latency.
The DC P5800X is among the most expensive drives on a $/TB basis. That was the case when they were sold at list price and now that they’re only available in used markets the prices have not dropped.
Generally, yes, an Optane P5800X can work as a boot drive with the components you list. However, there is an epic thread about how difficult it is to successfully connect drives at PCIe Gen4 speeds that you should review. I cannot comment on the Delock or Startech products listed. They are spec’ed correctly, but the above thread indicates that this doesn’t have to mean a whole lot.
Once connected, any enterprise storage drive requires enterprise level cooling. Also, this can be available in a gamer’s setting, but is not guaranteed.