ian,
You should have 28 gpm if that is what you calculate for that pump and rpm, regardless of valve or hose size.
Fixed displacement pumps are, well, fixed displacement, and put out a constant flow if the rpm holds.
Small section plus large section is total output before the unloading pressure is reached. (21 + 7? = 28 output)
Small section is output after unloading the large section to inlet ( 7? output)
Valves and hoses don't change the speed, they only add pressure drop to the circuit. It still moves 28 gpm, just takes more energy to push it through small conductors.
Of course, if the relief valve seting is reached, or the unloading set point is reached, then unloading happens across a small range of pressure, maybe 100 psi say from 800 to 900 psi range if you are set there. Flow could drop off there, but I doubt you have several hundred psi drop through the valve. Hope not anyway. What is the pump or valve inlet pressure reading at high idle with valve in neutral?
kcj