I went with the middle model and put the bumper back on. Looks great! Is that the Alpina model?[/QUOTE
The bumper is very tight against the front wings. Is it possible to add 3-4 mm on each side?
Yes. The bumper is actually a three piece affair which I discovered by accident when I removed it. After installing the air dam, I was able to get the space you are refering to. There is some adjustment when re-assembling the bumper. I used a rubber dounut as a stand off (It's actually an engine mount for some sort of Chevy, available at my local NAPA autoparts store. You'll have to find an equvalent)
The bolts that hold the ends of the bumper are actually the most important for holding the air dam on. The sheet metal screws across the front only hold it in place without much strength.