Two thirds of a stop are a difference: even one third. It can be seen in wet prints.
Also, it's not the same 2/3 of a stop if we talk about over exposing an ISO400 film, than underexposing an ISO100 film, which has less room for pushing from a film design point of view.
Apart, developers react differently to that... With 2/3 of a stop below what Perceptol needs, we lose all of that developer qualities.
Trying to get a good 160 from TMX requires metering skills: it's a more precise task than trying to get 640 from HP5+. Or than exposing HP5+ at 250...