Yellowjackets are omnivorous, meaning part of their diet consists of other bugs. One thing radiators are full of are dead insects which are easy to forage. This also explains why heat doesn’t play into their activity. Your best defense is a good wash.
The plenum is usually an enlarged section feeding the 4, 5, 6, 8 or more ports so that there is less turbulence in the air as it enters the ports.
Too much turbulence and funny things start to happen like one cylinder runs rich compared to the others.
Sounds like you have replaced most items suspect of the cause of overheating at high speed. The return hose from rad to water pump may be collapsing when water pump reaches high speeds. This will cause restricted flow of coolant hence less volume therefore overheat of block fluids. If you have been around vehicles as long as I have you may have removed "...