how did they confirm the restriction? the way we do it at my shop is, when you are looking at actual values of the Mv of the oxygen sensor you first activate the air pump and see where the mv drops to, if it is not down below 40mv within 60 sec, you can remove the brake booster hose to create a vacuum leak which will simulate what a normal air pump does when on. by mercedes standard it should drop below 40mv with in 60 seconds. normal system will drop to under 20mv, to 10 or so. the tests require factory tools, using it's activation feature. this is what they should have done, you may want to call them and see what they say. the code could be simply an o2 sensor, vacuum valve, or even cracked vacuum lines. i am not saying they are wrong, just for that price it pays to do your own investigating