People tend to forget the first role of the air filter. It's is to FILTER air. Some of the higher flowing ones don't filter out as much dirt in the air, but give high HP readings. Restrictive ones don't give as much HP.
There's no need to spend big bucks on air filters because they give only a few KW's of difference, which you won't feel!!! Who can tell the difference if their car has 0.8kw more?
From experience, the HKS flow good, but it lets too much dirt get inside. Plus the filter paper is not re-usable. The Blitz filters are expensive and also let in a far amount of dirt. The best one I've used so far is K&N, but just be careful not to use too much oil when re-washing it. Don't know much about the rush ones though.