Don't do it.
Think of all the money K&N have spent on research and testing - It's more likely the slander is either from trollers or from people who haven't got a clue what they're talking about, although I have seen Dyno figures that show no/little improvement over stock filters - But they do sound good, right?!
Secondly, the 'B&Q foam' will not be up to the same quality, I'd be worrying about the foam element breaking down and causing all sorts of nuisance.
Lastly, don't do it.
Don't, just No.