I'm a newbie to AO, but used to know the guy who runs GSR (not in touch now, but at one point I was helping him with clubFIREpower - an owner's club for all FIRE powered Fiats/Alfas/Lancias - admittedly the club didn't quite go to plan).
Going back 5 years or so, I had the GSR kit on my Seicento Sporting (it had a Punto 1242 8v engine in it - a common mod). There's no doubt that the BMC CDA filter is doing the lion's share of the performance boost - but the silicone is there for a number of reasons.
First, the tooling used to mould the silicone can be created in very complex shapes - properly radiused elbows, reducers, take-offs - good for shaping the hose routing through the engine bay, and also to match the diameter of the clean side of the filter to the intake diameter of the manifold/MAF sensor, which might be different. The Seicento version didn't have any take-offs, but the Alfa ones do.
Second, it's resistant to chemicals (such as oil, fuel, coolant) and is a good insulator - keeping the incoming air as cool as possible is always a good thing (those small FIAT FIRE engines got particularly hot, and the air intake tract was directly above the exhaust manifold!).
Third, (and this is probably the contentious one, having read a few posts here on AO) was about smoothing the airflow. I've heard people comment that it was about the silicone being smooth on the inside, meaning that the air flows better. I'm 99% sure this wasn't the reason - silicone gives you an easy way to mould smooth transitions of hose diameter, removing any steps or sharp turns in the airflow. Same word 'smooth', but a different meaning.
I 'DIY'd' a BMC CDA kit for my Eunos Roadster 1.8 - made the car sound gorgeous on open throttle and gave noticeably more part-throttle response under 4k revs (where the stock intake seems to behave pretty poorly). All I did was attach a short reducing-diameter silicone hose to the air side of the MAF sensor, and a second-hand CDA filter to the hose, leaving the stock resonance chamber in place. £65 all in, and from a 'seat of the pants' perspective, a great improvement!
I'm babbling now - bottom line is that the CDA is a fantastic performance filter. That Top Gear episode must have got me all worked up.