I believe what really matters is where the intake ingests air from, ie how cool the source and how air turbulence is controlled in the intake tract. The air is traveling so fast that the metal tube is only an issue at idle and crawling speeds. The problem with a Y or T is how the air crashes entering the manifold. For instance, in the case of some V-8 cooling systems, the connection of the 2 cylinder banks in the intake manifold before the thermostat will flow one side at a time. The hotter cylinder bank, or side with the most pressure will flow until the pressure or heat from the opposite side is greater. So you have a situation where its one or the other cycling back and forth constantly. I don't know much about air dynamics but no matter the size of the tube or tubes, the transition into the intake is important. I like the single tube for the dynamic and harmonic control of the air, and the better designs will have taken this in account. Velocity is important also. If the air is crashing in a dual design, it will hurt velocity too. I'm not saying a dual design is bad or inferior, just it's real hard to make it do what one would like it to do i.e. be better than a single tube with a well thought out filter location.

Les