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I would guess that, because the grill is located within a flat, front-facing profile of the bumper, the 1-2 inches the grill block is recessed will not add any quantifiable drag. That flat section will create a high pressure/low speed pocket of air around which the surrounding airflow will divert.
Again, this is my guess, but the small recess of the grill will not measurably effect how air moves under or over the front edge of the bumper. But I could definitely be wrong!
However, mostly for a different reason, I am working on a new grill block that I will actually attach to the front of the bumper instead of within the grill. Along with this I am making a rectangular duct that will extend from the grill block to the edges of the radiator in order to maximize the efficiency of the remaining grill opening.
Also, I've since extended my front tire deflectors inward to divert air around the wheel wells, I'm working on a new section of the undertray that will add more coverage, and I also made small rear tire deflectors.
I've not had any melting/warping issues with the undertray being close to the exhaust. I initially lined the edges of the undertray where the downtube is with aluminum flashing, but discovered there really wasn't much heat build-up, so I removed it.
Anyway, when more pics to come soon!
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