Seems to me, you want to keep air from becoming trapped under the nose, which then lifts and/or jams under the draggy underbody of the car. Hence No. 6 above has lowest drag, lift, and power consumed.
It also seems to me that although the Miata nose planform (as seen from above) could be improved somewhat with a more elliptical shape to make for a smaller stagnation point which sheds the air more efficiently. In other words, droop the nose a bit more, make it more pointy/elliptical, then add the dam/splitter with no concavity.
Start with this and then make those mods:
https://encrypted-tbn1.gstatic.com/i...bVOwUHZqImq5wm
The Miata numbers posted above do not include an undertray, but I'd wager that an undertray extending aft from the plane of the splitter of No. 6 would be even better, as would extension of the air dam further laterally to completely shield and fair airflow around the front tires. Finally, using ram air from the stagnation point intake, duct it to the front wheel wells as a BMW-like air curtain.