i believe the high flow and high pressure is just refering to the pump, not the complete system (engine)
i think with a gear pump, the pump curve is a fairly straight line on the graph, so flow relates to rpm, and as snapper says the high flow pump is taller, so does give a higher flow into a given system for the same rpm
the relief valve is set higher, than standard, yet it will only reach this when given cold oil, and an engine whose bearings are tight enough not to relive more pressure than you are generating for a given rpm
basically if you have an engine thats not on its last legs the standard pump should be more than adequate
thats how i understand it....