This is the hose reserved for a positive crankcase ventilation valve (PCV) found on most "normal" engines (not Porsche though). This hose contains normal (and abnormal) blowby gases (unburned fuel) from the crankcase and whatever oil fumes the blowby takes with it.
Your hose route back to the individual air cleaners sounds like a good plan. However, I'd suggest installing some sort of catch can between the factory oil filler pipe and the air filter connection. At high engine speed, there can be quite a spray of engine oil from this hose, more so if the rings are a little on the weak side (more blowby). The catch can helps separate the oil from the fumes so only fumes go to the carbs and intake. I used a plastic windshield wiper bottle with some PCV fittings from the local auto parts store to attach the hoses to it. If you track your car, you'll probably need some type of container at the end of this breather hose. I've seen a race car with an 8 oz. Minute Maid orange juice container installed for this sole purpose.
BTW, recirculating the blowby does contribute fewer unburned hydrocarbons into the atmosphere.
Sherwood Lee
http://members.rennlist.org/911pcars
www.seinesystems.com