Thanks everyone for the clarification on what's going on inside of that T
I will try my best to give you accurate result of before and after the installation (on my car, wich is now in fairly good shape). But I don't think I will be able to dyno it :/
as I said, the main advantage of my plenum over the 997 plenum (which I recon, is a good deal), is the perfect fit "as original". Is it worth it ? I think it does !
Ben.