Take care with filters like K&N's (and other free briething foam types) They are not known as "Rebore filters" in some circles for nothing.
With a turbo too, the fine dust that will get past it, will damage the compressor vanes of the turbo, as well as accelerate cylinder and piston ring wear...
I can't find the link any more, but there was a briliant article on a US 4x4 site, about filters in general, air filters in particular, and the reasons for all the different types.
Most of the foam types were just to keep big stuff (dead leaves etc) out of racing car engines.
Yes you can spray the things with sticky filter goop, but if you drive the thing regularly, you need to keep washing and re-coating the filter each week or two.
Been there, done that, rebuilt the engine, went back to paper (if oversized!) filters much less fuss and cost, long term, and cleaner air for the thing to breathe too.
Dave B.
200TDi with a V8 paper filter. Well, at 1bar boost, the engine takes as much air at ambient, as a 5l V8 would! Confirmed by several race types I know. And after nearly 20k miles, the turbo inlet is still like new...
