When altering the factory air filter make sure that the filter is able to trap the dust particles, as some aftermarket filters will allow a lot of dirt to be swallowed by the engine. So the short 0.5kW power increase will be forever lost (with a big interest) very soon when your engine is prematurely worn out.

Those filtering efficiency figures quoted can be very tricky, since they don't say the micron size of the particles the filter is able to filtrate out.