Years ago with a BMW K bike, someone did a test to determine the potential of more free-flowing air filters. What they did is simply cover half of the stock filter with plastic or paper, such that the active flow area was half of normal. They then did a dyno test with "half a filter" and normal. There was no HP difference between the two, i.e., the stock filter flowed so well that "clogging" half of it had no impact on HP. With those results, it is difficult to believe a more free-flowing air filter would have any impact on K bike HP.
It would be interesting to see a dyno comparison of an X5 with the stock filter and with no filter whatsoever, to more precisely identify the absolute maximum potential, if any, of reducing air filter resistance to air flow, without making changes in other areas (figuring no filter has the absolute least air flow resistance). I suspect the answer might surprise many people.
P.S. Here is a link to the only independent test of air filters I've come across. It doesn't address HP gains, but does show the trade-off between restriction and efficiency, at least for these particular filters:
ISO 5011 Duramax Air Filter Test Report
01-27-2011, 03:50 PM
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