I have two loops coming off my res.

Subloop1: res -> pump1 -> pump2 -> pump3 -> pump4 -> block -> flowmeter1 -> flowmeter2 -> res
Subloop2: res -> pump5 -> rads -> flowmeter3 -> res

This way the flowrate through my radiators is always constant, no matter the flowrate through the block. Overall, with 3 pumps 'off' and 2 flowmeters, my block-subloop restriction is fairly high, higher than a basic tubing + res + rad + block set.

Mathematically speaking, ancillary components should create a ceiling for flowrate, not have a floor for flowrate (i.e., the effect of the restriction of the block diminishes). Because HF-p4 is getting a higher flowrate, his (Jay's) secondary components are not creating a ceiling and his numbers say that GTZ ~= XT ~= HF-p1 in restriction. My guess is a fairly extreme variation in production quality may be at play here. Maybe the machining on his Supreme HF has resulted in shallow microchannels (relative to spec and/or mine). That would reduce surface area, decrease cross-section of the flowpaths, and increase the distance between the water and the IHS (all in all creating a noticeably negative effect thermally and increasing restriction).