[euromodder]

You might get away with blocking that much in winter, but I fear it'll prove too much in summer.

If your car has a cooler for the transmission oil, make sure it still gets plenty of cooling air.

Smoother is marginally better, but probably not by enough that you'll be able to tell the difference in daily driving.
That's why OEMs block the grilles on the inside - making you wonder why they keep the grilles all together, but that's the marketing and brand image boys having their say.