Have you ever thought about what those tube-like PVC-structures attatched to the OEM intakesystem and airbox does? According to my sources, they grant for better torque at midrange rpm. And thats why the engineers bother with designing them, I guess.
This week, I`ve experimented with a K&N 57i, my friends stock airbox, and my own modded OEM airbox.
Here is what I experienced:

1. K&N. Nice bling factor. Nice sound. Flat spot from 3K to 4.1K. Possibly more hp form 6.5K to 7.2K.
2. Standard box. Oem look. Silent. Good torque at midrange - no doubt!
3. Modded airbox. OEM look, same sound and characteristics as K&N.

I was hoping that my mod (perforating the narrow bottom-rear part of the airbox for better airflow) would outperform the K&N since I kept the resonance structures, but it did not. I lost the midrange torque.
I`m not sure if the felt increase power at top rpm with mod or K&N is real, or if it just feels stronger because the midrange is so flat.
Anyway, my friend will not use his airbox after buying the K&N, so I choose the stock one.

Tormod

 

One of the gurus here mentioned a few weeks back that Mini had improved the standard air intake in '04 or '05 to the extent that there wasn't much to be gained in that avenue. Your experience is consistent with the problem I have with K&N and other open element style intakes - they make more noise (pleasant or unpleasant depending on your perception) and any real power increases they make are marginal, and often so high in the rev range as to be irrelevant, and sometimes at the expense of more usable midrange.

 

I can't understand how those hollow appendages on the intake path of most modern cars could possibly affect engine torque. I think they are there only to disrupt standing waves in the air stream, thereby reducing noise at certain air velocities.

 

I suppose it has to do with the pressure wave created by the intake air that had been flowing smoothly into a cylinder suddenly coming up against a closed intake valve. That pressure wave then runs back through the intake tract and can interfere with the normal flow of air. At a certain RPM, based on the engine displacement and the length and volume of the intake tract, a pressure wave will resonate with enough violence to restricting airflow. The (anti)resonance chamber in the stock intake tract absorbs the pressure waves before they cause problems, i.e. at a certain rpm the frequency just vibrates back and forth in the chamber without leaving.
I`ve not seen dynos for the Mini, but in the late 90s, there was dynoproof that Naturally aspirated Miatas lost 4 ft.ibs of torque with aftermarket intakes. When we turboed them, no one cared of course
You should try by yourself, I certainly don`t need no dyno to tell the difference on a Cooper.
Tormod