Chin Splitter?
 

Does a chin splitter (something along the lines of this:

http://www.rpidesigns.com/images/zr1splitteroncar.jpg

Help for MPGs or downforce? I can't remember which... or is it both?

I'm thinking if it will help with MPG, then when I make my belly pan, I'll extend it out the front.

Improves downforce 4 sure.
I don't know about MPG.

It improves downforce through the wing effect. It can aid MPG by lowering your stagnation point, if you mount it correctly. It might not be a noticeable gain, though, certainly not worth buying one premade.

They reduce lift by preventing air from going underneath. That also reduces drag. The two aren't mutually exclusive.

Mount it at the wrong angle, then get back to me about how it doesn't induce drag, rather than reduce it.

Usually they're horizontal though

That would be a chin "spoiler"

http://www.atlantaauto.com/imagesv3/large/3d691071.jpg

Here's a more dramatic chin splitter... which more than likely does no more than a smaller one. (also called a front diffuser)

http://ll.speedhunters.com/u/f/eagam...Grip/yf141.jpg

The splitter is horizontal. That Mustang has an air dam. The dam reduces drag and lift by the same method though, preventing air from going under the car. The difference is that the splitter doesn't block airflow coming from the front, only airflow being redirected downward by the nose of the car.

Ohhhhhhhh my bad. I was thinking you said vertical.

splitter
 

My opinion,is that the splitter's place is on the racetrack,not on the street.
Neither the Corvette pictured,nor the racecar could navigate a driveway ramp without leaving their nose on the ground.
The Corvette,which can be purchased in Germany,can legally operate on portions of the Autobahn at top speed,which for some models now is in 200-mph territory.
Stability and traction are paramount to safety at these speeds,which the splitter can help with.
For normal day to day driving they would offer only a headache and hazard to surrounding traffic as you slowed to a crawl and attempted to negotiate ramps obliquely to avoid a ground-strike.
I've been there,done that,don't go there!