I may be entering this conversation late, and I know I don't have a 5.0
BUT I think some clarification is needed. And basic engineering prinicples.
An engine will require an air flow rate of X amount. If the standard airbox setup can supply this amount of air flow then any CAI and filter combo will not help in this area. As an engine will only ever flow as much as is needed. If the stock air box is restrictive (V12 Jaguar XJS for example) then a larger filter/intake path will help as it will allow a greater flow to the engine.
To visulise, imagine at WOT (wide open throttle) and engine requires 1 pint of water, if the glass is to small it will not be able to hold enough water, if it is bigger than a pint you will not see a benefit as the engine still only wants a pint, the rest of the glass would be empty.
By changing items such as exhust and cams and better flowing heads and things like throttle bodies you can make an engine require a greater air flow (using the above example it would know need 1.5 pints) so the stock 1 pint class can no longer supply enough. A TB change on its own may offer nothing if the intake can not flow sufficiently for it, OR the engine does not require the extra flow rate due to limitations in cam profile, head flow, timing, etc.
Some cars will have restrictive airboxes in stock trim, some only once the car is modified. Yet some still maybe ok even for heavily modified engines.
More proof is 'air flow restrictors' as used in rallying and racing (such as GT racing), this simple item limits how much air can be flowed into the engine, thus limiting how big a bang can be made in the combustion chamber, limiting the amount of HP produced. This is done so that cars like a Ferrari F360 with 3.6 litres can produce the same BHP and compete with larger engined cars like the 5.7 litre Corvette.
The only way to get more air into an engine is through Forced Induction (FI), this increase the charge densisty, not the flow rate. The engine will still be requireing 1 pint glass of water, BUT that 1 pint glass would contain 2 pints of water due to it being compressed.
The so called RAM air hood scoopes offer little gains, because as explained above the engine will not increase its flow rate above what it can handle, the only gains a hood scoop will make is at high speed where the air hitting the inside of the scoop will be slightly 'compressed' increasing the charge density, although I doubt you will see any gain this side of 80mph, and only a very small gain at speeds of 150mph+. A hood scoop will also cause drag and make the vehcile less aerodynamic with a greater drag CO.
Another take on ram air is the ram pipes (bell mouths) inside the intake manifold, if these are harmonically balenced they will aid in air flow causing less turbulance (in the same manor as a note is played from a brass instrument like a trumpet). Again the only benefits you will see is if the stock setup is restrictive to begin with.
As for the ducting, yes a smoother less turbulent setup should in theory help, but the gains will be so limited and the rest of the system would need to be optimised to take any benefit. The material will probably have little affect either, as the air moves very quickly along it, the only time it may be of concern is on a very hot day with a heavily heat soaked engine bay. BUT once you start moving the air should be cooled down to ambient temperatures anyhow. An as you will not be able to reduce the temp below ambient it will make little to no odds what the tubing is made from.
The rule about air temperature is simple, cool for power (maximum charge density), hot for economy (minimum charge density to reduce losses due to throttling). With regard to the V12 XJS the standard arrangement is much better than many people think. Sure, the under-bonnet air temperature at idle can easily get up around 70 C but the faster the car goes the lower the air temperature falls - simply because the radiator is passing its heat to a much larger quantity of air per second - so at 80 m.p.h. the engine is breathing air at around 45 C. That's still a bit higher than the ideal but not nearly so bad as many people think. This principle is the same for ALL internal combustion engines.
As for the shape of the tubing, well the air is being drawn by the engine, not pushed from the out side, so bends (providing they don't reduce the diamter or cause turbulance) should not affect the air flow. To try it out, get your vacuum cleaner with the pipe extension on, switch the unit on. Holding it out straight (no bends at all) then turn it through 180 degree bend in the pipe. It should still be sucking at the same rate, there should be no noticable difference.
As for the dyno runs, well to be completly impartial you would need to compare a fully functional factory setup (ie not an old dirty filter and a possibly leaking air ducting setup) to a new CAI. If you have a 5.0 it will probably be a good 15 years old by know, so by replacing an old and dirty (blocked) fliter with a new cone setup should see improved performance, but possibly no more of an improvement over replacing the old filter with a new stock one. Tests done on this basis are NO proof that a CAI works on a stock 5.0
I am not say that a CAI will not yield better numbers, just mearly the possibility that they will not be any better than a stock setup on a stock car. If you want one, I say go for it. If not don't worry and buy something else instead. Because even if it does make a difference it will only be a small amount over good (ie clean/new filter) stock setup.
Hope this helps.
