There was another aero thread where I think Tony D nailed the reason why John's splitter didn't work out so well. He had no airbox, so essentially a whole lot of air was getting shoveled into the engine compartment, and his was getting scary light in the front end at speed. If you really want the splitter to function, I think the rad duct is a necessary part of the equation.
Another option is what a bunch of the wind tunnel test guys did (bjhines comes to mind), which is to add a flat plate from the air dam to the rad core support. Simpler to construct, less obvious, strengthens the airdam, and doesn't get bent or torn up if you drag it on the ground. It prevents some air from going between the air dam and the ground under the rad, but doesn't prevent it from going over the rad, so not nearly as good an aero solution as a rad duct and splitter IMO.
Edited by JMortensen, 18 March 2014 - 12:52 PM.