Hi,
Icksnay on the AirTabs-ay -- I doubt they will help, and they probably would hurt your FE. I've tried them, and it was a waste of a fair bit of money...
Here's my sketch of the grill block wedge I'm suggesting -- it's similar to mine:
Some more air may flow to the sides, though probably the difference is subtle. It also would not block (the little bit of) light from the inside edge of the headlights. The blue lines are foam gasket or tape or caulking to seal the air gap -- the front edge of the hood is the more important -- the air flow velocity there is higher and keeping it out from under the hood is important.
Here's my Coroplast grill block:
Your Legacy has less curve to the sides (along the brown line on my drawing) so shedding air to the sides may be less important than on my xA.