I contacted the seller who is a performance workshop as well as a seller of performance gear and he said the install/tuners simply put the big end on to the airbox, it's a perfect fit there, and the smaller end goes very tightly into the intake hose and because there is no pressure in those pipes, just a vacuum, there is no need for an alloy or whatever sleeve to give them strength, the double 5mm layers of the 2 hoses is more than enough and they just put a clamp on the outside ..I had already tried that and it is a nice tight fit and it wouldn't come apart as there is no where for the two hoses to go if they spread apart.. and like he says, there is no pressure in there..There is no such thing on ebay that is 80mm inner on both ends.. It shouldn't interfere with the airflow because it is no different having the hose go inside the big one because it is no different to the airbox going inside the fat end of the pipe.
How novel. if it works - fair enough. If you're nervous about the rigidity a short piece of 70mm pipe on the inside of the two overlapped silicone pipes should give sufficient rigidity for the external hose clamp to tighten down on.
I have also been investigating the need for bigger injectors, stronger fuel pump, and a pod filter for the extra surface area and the general consensus is I don't need any of that because It's only going to be making between 180 and 200 and all the bits already on the car stock will cope with that. The K&N panel filter handle 480cfm and a vf52 maxes out at 420cfm.. It wouldn't cope on my rex because it's VF22 makes 490cfm . It has a proper inguard pod setup. It makes a nice induction sound that's for sure.
I would tend to agree - don't bother with those extras. "Mission creep" is an expensive time-consuming beast at the best of times for car modifications.
Modest increases injectors and fuel pump are "ok" but there is definitely such thing as "too big" for each of those items. Eg:
(a) "too big" injectors lose fine control and idle and similar low-load, leading to worse fuel economy, rougher idle etc.
(b) in my experience a "too big" fuel pump caused fuel-return-line (or regulator) capacity to max-out on a cold start (where the fuel pump runs on the high setting but the injectors still have a small pulse width), causing awful fuel-smelling exhaust on start-up. The solution was either a more modest pump (selected) or spending even more on upgraded fuel system to increase the return-to-tank capacity.
The stock airbox does its job very well for the range of power available from Subaru-supplied turbos (including the VF52).