Apologies - I skim read the OP's post. He's having it rollered - doing it properly. Many don't and disappointment ensues.
So fitting a shorter inlet tract will give you more torque vis a standard filter box+element. Afraid not. That's why the most efficient manifolds put volume into the inlet plenum. 5-5.5" long runners on Weber manifolds to boost torque.
I've fitted K&N filter box and stub stack to similar modded set-up, 1275cc Maniflow LCB Manifold, RC40, modded head onan otherwise lightly modified car. It also ran SU HIF44 so I am talking from personal experience and qualified to give some constructive comment. Otherwise I'd have just read the post and STFU.
I've just fitted an ITG filter to my 45DCOE, they are brilliant bits of kits so I'm knocking personal choice either. Its just neither the ITG nor K&N will give anything like 4-5 bhp - utter madness to suggest such.