You wont lose much power if any at all by keeping the air box. I managed to get just over 63bhp out of mine with an air box fitted that was very slighty modded.

I tryed playing around on the dyno with and without lids and with parts hacked off. I ended up running just 2 x 1" holes cut into the lid roof. This gave the best results. The best i saw without an air box was 61bhp plus it took an age to sort out the mid range fueling. Im sure ive gone over this before but engines love large volumes of still air which is exactly what an air box gives.
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If I have to take the carbs off once more...