It wouldn’t really do much good to re-route the intake through the A/C ducts because the volume of cool air needed won’t be available.  Engine intakes require a significant volume of air, much more than any A/C system can generate.  Anything you put between the atmosphere the intake manifold will restrict airflow and thus offset any performance gains that cooler air might provide.   Of course your air filter restricts airflow, but it’s better to lose some performance to it rather than to ruin the engine with airborne particulate. 

Many turbo and supercharger systems use “intercoolers”.  This is important because those systems pressurize the air, which also causes it to become heated (refer to Boyle’s Law).  Instead of the intake air being a few degrees warmer than ambient because of engine heat, this charger-compressed air can be 200F above ambient.  The intercooler is a radiator-like heat exchanger that uses either ambient temperature air or water for cooling.  Of course this does restrict airflow, but since the charger is pressurizing the air in the first place, it will still reach the intake manifold with positive pressure, overcoming the vacuum demands of the engine.  Note that an intercooler uses either ambient air or water to cool the intake air, so it can’t get any cooler than ambient temperature, and is in fact still going to be warmer, but it will be much cooler than the compressed air coming out of the charger.

So-called “cold air intakes” don’t actually cool the air any lower than ambient temperatures, either.  They just attempt to insulate intake air from the heat under the hood.  Some systems, like K&N’s, claim to have special filters with better airflow properties than OEM filters.  There is plenty of data from both sides of the argument to totally confuse even the most savvy gearheads.  My opinion is this:  Mercedes-Benz has been making automobile engines for more than 100 years.  In that time, they have done some seemingly wacky things to squeeze extra performance from their engines in order to move their very safe, but also very heavy vehicles, down the Autobahn.  If they thought that an extra 5-10hp could be safely gained by using a K&N filter, it would be standard equipment.