Be aware that directional lavs if mounted off center on the talent that head turns will cause a drop in level also directional mics are more prone to plosives from p-pops and breath blasts. I would suggest that you try one out as a demo from a dealer with the right of return if it doesn't work out for you.
If ambient set noise is a major issue then consider an automatic mic mixer that will open and close the active/non active mics as required. OTOH controlling set noise should be your number one priority. Remember it's not a sound problem, it's a location problem.