I dont think new Cats will fix your problem either. Maybe they will lower the numbers down a little but not a whole lot.
Nox = Nitrogen and Oxygen combined at high temperatures, too much Oxygen in your system.
NOX too high is from too high of temperatures in the combusion chamber - or your engine is running too hot alltogether.
Usually the EGR lowers the temp by adding extra air ( burned air has less oxygen in it, the hot air displaces the oxygen in your engine ) but your car does not have EGR.
Timing could affect this, but timing is non-adjustable on these engines also.
You could also have a vacuum leak making the engine run too lean. I would check for vacuum leaks first, then make sure your o2 sensor, MAF sensor are working good, and Check your Fuel pressure also, too low of fuel pressure will make it run lean also.
- Dan