Interesting thought. Limit of 50 pax/CC means 199 pax = 4 CC, as HPbleed said. But somewhere in the back of my mind are undercarriage and wing limits for the A320 which mean that the A320 Plus might be rather different.for the 200 pax.
And watching the pax load/unload process on the A321 is quite painful - the single aisle really slows things down. A 200 seater is not necessarily better than a 180 seater. The scheduled turnaround time has to be longer, which changes the economics.