Hi jthistle,
I think you misunderstand the value of wind tunnel (or other sources) findings for coefficients. On the front of the car is already some structure, replacing that will alter the drag by almost exactly the ratio of the previous Cd to the replacement's Cd. If the grille had a Cd of 0.2 and the replacement grille block is just a flat plate it has a Cd of about 1, then the additional drag will be about 5 times greater than the old grille. If the replacement has a Cd of 5 it will be 5 times worse than the plate. Now we can't say exactly how much drag the original grille produces without fancy testing, but we can say that other structures will produce more or less drag proportional to the change in Cd.
Coast down testing is a quick and easy way to test, especially grill blocking. I look forward to your test results too.
Here, California98Civic says,
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Drag is important, and it is our normal obsession. But I would trade marginal drag increases in my grille blocks if it meant significantly reduced radiator-fan run times.
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I know what you mean. We all want to know, and in particular for our own cars, which modifications save fuel.
-mort