My girlfriends last car was a 318 compact m sport. The motor was actually a 1.9 and unbeknownst to us, the previous owner had slapped an exhaust off an 1800 on the car. I only point this out as visually, there was nothing to say it wouldnt fit, despite the differences between the 2 systems when you saw them on the charts.
But (and its a bigger but than vanessa phelps), we had nothing but trouble with that pipe. It was putting far too much stress on the joint at the cat end of the system. It blew on a regular basis. Successive attempts by the two garages she took it to in an attempt to stop the exhaust blowing and the cost of a new pipe only put off the inevitable need to replace it. The pipe eventually snapped at the joint. It had been bogged and welded more times than michael jacksons face before we finally had to replace it.
I only say all this as since that experience, I wont fit anything other than the specified part (unless its got something to do with tuning then all bets are off).