If you're starting to do aero tuning, you will want to get dirty... Buy an old-style "oil can pump," the type where you load it with oil to get at fan bearings, or hinges, or whatever. Next, squirt a line of drops across the top of the roof, and then take the car out on track for a session. When you get back in, you'll see where the airflow is. Repeat as necessary for the underside of the splitter, the top of the trunk, the leading edge of the hood, etc. This will give you a very good idea of where you have high pressure (oil is smeared towards the back of the car) and low pressure (oil stops moving, or hardly moves) areas, as well as where the separation zones are. There is little or no benefit to placing aero devices in low pressure zones, or in separation zones. Personally, I think there's a reason that the "race car" builds all run tall wings at the back. Also, make sure you research the difference between a wing and a spoiler, they're two different animals, and do very different things... In general, spoilers help to alter the flow plot to move low-pressure or separation zones to where they'll do some good, and wings create downforce. You have to get the wing up into the airflow, though, for it to be effective.
Also, going back to splitters, you may well see a better result from an air dam at the base of the front fascia than you will from a splitter that is survivable in a road-driven car. If you look at the splitter setups on the race Mustangs, like this:
You'll notice how close to the track that splitter sits, and how far forward of the fascia it protrudes. Also, note the tow point on the fascia! Granted, it's most likely used not for towing but for winching the car into a trailer, but it obviously has the structural integrity to support that kind of weight loading, which says a lot for the composition and mounting strength of the splitter. The second photo really points out the clearance needed for a working splitter. With the car either accelerating or near neutral throttle, the splitter is sitting about even with the bottom of the rim, and under braking, it's all but scraping the track, as can be seen on the #3 Mustang here, who looks to be trail-braking a bit, as well as on the following Corvette and Viper:
A splitter like that would NOT be survivable on a road-going car. The first speed bump, driveway incline, parking curb, etc. that was encountered would either high-side the car, or tear the splitter apart. It looks to me like the splitter is protruding somewhere between 3" and 4" forward of the fascia.
You'll also notice that as part of the full aero package, the cars are all running pretty huge rear wings for downforce, as well as extended sideboards to help maintain a low pressure zone under the car. Aero needs to be approached as a complete "system," to balance front and rear downforce at speed, so if we deem the splitter functional, it apparently takes that much wing to balance it out.
