Have you ruled out an air-fuel ratio issue? If it's running lean you'll never get it to run cool.

A splitter is legal, as long as it conforms to the regulations.

I'd like you to get some advice from a 1st gen guy, because there are some significant differences in location/mounting of some key components (radiator, for instance). I use the stock cooler mount below the round bar for one cooler, and clamp mounts for the 2nd cooler to the round bar, with the cooler positioned above the bar. A sheet metal divider forces air through the bumper cover into the upper cooler, while air below the bumper cover is forced through the lower cooler. All the air from both coolers then is forced through the radiator. It took some custom sheet metal fabrication to get that all sealed up. I've seen other arrangements where the air though the lower cooler exits out the bottom and bypasses the radiator.