Unless the original buyer specifically ordered the Mtech springs and rear stabilizer bar (14.5mm compared to 12mm) the 4-door and 2-door 318i/iS came with the same suspension setups. The only difference in the springs was if the car had A/C, Sport package, or sport package with A/C. The same 2 sets of springs were used for both 4 and 2 door models. Check the ETK if you want to see for yourself.
BTW are those curb weights definately from the US market cars? The european e30s were all lighter because they didn't have some heavy metal "reinforcement" bars behind the bumpers.
The 318iS also had a few things like the rear spoiler, the front skirt, and the limited slip diff (for Canadian models for sure, i think it was option in the US). Anyway, there were different standard options for cars sold throughout north america so i would only use those numbers as rough estimates. Maybe the 318is weight listed there has a sunroof and the 318i didn't, who knows.