Just returned from a little dealer sponsered "auto-cross" session. I'm sure your familiar with this, parking lot with orange cones. The auto-cross cars were one 328i sport with auto and one 323i sport with auto. I don't know how long the course was, but it was quite short with an oval style shape with one chicane on the back stretch. All four corners where slightly different. The lap times are about 16 seconds, well to make a long story short, my best time in a 328i was 15.89 and my best in a 323i was 16.43 (both the quickest of the day thank you). On such a short and slow course that's a big difference. Make your own conclusions. Personally I give half to the 17" tires and half to the 300cc. Which probably means the 193 bhp is somewhat conservative (note: 193 bhp DIN is an insurance threshold in Germany, just like 280 bhp is the "limit" in Japan by agreement, yeah right 280 for a Supra and Skyline).
I would bet money however that given a rela track where the 2.8 L low-end torque won't be such an advantage the difference would not be so profound.
Moral of this story: A 323i with 17" rubber is probably only slightly slower (though you do give up some ease of driving due to the 328i's good low-end).