My old car didn't have AC, so I was able to block half the grill with no fear of overheating, so that's where I started. At the same time, I ducted the flow from the remaining opening into the radiator, so that the air that was coming in would be forced through the radiator. I also put in a belly pan under the engine bay and one from behind the rear wheels to the rear bumper that acted like a diffuser. The gain from this was ~6% better FE, and a LOT less road noise. Once I was comfortable there, I started blocking off more and more of the grill. In the end, I had just a very small opening for winter, and a slightly larger opening for summer. You can see pictures of my mods
HERE.
Since you have eliminated the AC, you may be able to follow a similar course of action.
I also did some more work on it,
adding a kammback, I don't expect you to do so, especially if you want to keep your stock look, but you may be interested.