Reason the thread hole looks small on paper for highflow is because threaded holes are based on radius, whereas tubesize is based on diameter. A 1/4" BSP threaded hole has (more or less) a 1/2" diameter...
3/8" BSPP is largest currently in use on radiators, and is also a common size on 240v pumps such as the Eheim1048 & 1250, but there are many pumps out there that use 1/2" BSPP fitting threads (yes, that's a 1" hole)...
With 1/4" BSP threaded holes, you're limited to a maximum realistic hose size of 1/2" ID, whereas 3/8" BSP allows for those who like to use 5/8" ID and larger tubing.
There are some extreme watercooling people out there running 3/4" ID hose, which is only of any benefit if it's connected to something with a 3/8" BSPP hole, otherwise the hole creates a restriction which negates the benefit of the larger hose.
... I guess you are going to be a good person to talk us through the watercooling world from what I can see. That's great.
