Thanks for the Beetle link haha.
The splitter actually helped out with fuel economy as far as I could measure. The stock front end fore of the Skyactiv undertray is pretty crappily designed, so the splitter actually filled in all the gaps and left just enough room for the Sky undertray to slot in. This pic of the splitter bracketry should explain my case:
The splitter covers up all that gap BS and lines up cleanly to the Sky undertray. Between those two parts the entire front end is flat to the front subframe.
From the front subframe back, there is room for improvement as you can kind of make out from the previous pic. The floorpan actually has flat cover-panels from the factory, but the exhaust tunnel and fuel tank area are exposed and that's where I have gains to make.
Aft of the rear subframe I'm also in pretty good shape as I homebrewed an Evo IX Voltex diffuser and aluminum "subframe" that covers everything up to the exit. (There's transparent Lexan in the empty space, may not be visible)
So that's where I stand at the moment, hope that clears up what I've already tackled/thought to address and what I have yet to do that I should. Again somehow the car is consistently reading 37mpg highway and it had not done that before the diffuser AND splitter were installed. Since they both make usable downforce, I'd like to keep them and refine them for lower drag rather than remove them entirely.