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I'm looking at the 2nd edition, so hopefully this material is in yours too--
I don't think you're missing anything. There are two competing aims here: first, to improve the radiator's capacity for dissipating heat and second, to minimize drag of the inlet/radiator/outlet. The dissipating capacity of the radiator is given by
Q = K1 * A(Tw - Ta) * V2^(4/5)
According to that, it isn't simply a case of slower air into the radiator improving its ability to cool; the dissipating capacity goes up with speed.
Greater pressure difference across the radiator should improve its cooling (because you're moving more air through it), but to get that you either need to slow down the air entering the radiator (losing momentum --> drag) or speed up the air exiting the radiator (accelerating --> drag). This is summed up in the equation S-R gives for the drag associated with the duct:
Dint = (Vi - Ve)(dm/dt) + (Pi - Pe)Ae
where the first term represents the momentum loss through the duct (change in velocity times mass flow rate) and the second term the "work required to overcome friction and internal obstructions" (change in pressure times outlet area)--for example, a radiator in the duct. The word "work" here is a bit misleading; pressure times area gives force, so this is the force required to do the work (energy) to overcome friction and obstructions in the duct.
Then you have to throw in the fact that all these equations are derived from considering an ideal duct, which is very much not the case on our cars (unless you've fully ducted the inlet and outlet and avoided radical direction changes!).
So, short answer: yes, measuring the difference in pressure across the radiator will be your best measure of its effectiveness (as JulianEdgar has shown in Autospeed articles), but if you're looking to optimize both cooling and drag from the radiator, you'll want to measure overall drag as well to see if there are any measurable changes using throttle-stop testing or another method.
My two cents, anyway, and hopefully my summary of S-R is correct.
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