We all remove the bottom one but all leave the trumpet that feeds the MAF. Think it was Nutron who took his top one off on a rolling road session and his performance became worse with it removed. I think its there to speed up the airflow so affects torque?
I've not noticed any difference performance wise since removing the top one. I may see if I can get another top half of the air box and see if it does feel any different after driving around for several months since doing it.