you might want to have the car's AFR checked on a dyno, preferably before the cats, because these cars where detuned by Mercedes and run very lean, so you might need to get the ECU reflashed with the slk32 programming by a performance shop soon!!!!!!!!!!1

also NA cars are very lean runners and need larger injectors and a tune. MBZ probably used the C230 injectors on the NA 320. I know if I run it hard for 20 seconds in 5th gear the catalitic system gets real hot and gives off the tell tail rotten egg smell.