323ic single exhaust is not restrictive for 2.5L engine. It goes down to 2 1/4 inch in front of the converter, after the converter it goes to 2.5 inch wide pipe. 190 horsepower has no problem with 2 inch exhaust pipe.
328i and M3 mid section is worse. Right after converters the two small resonators, the pipe is only 1.5 inch wide inside on each side