[Jaime]

The air box isn't restrictive, but we must remove it if we want to use an universal filter. You can check this removing just the filter and running a bit; you'll get the same unusable bike than also removing the air box.

So, modifying the original filter is a good option too and requires fewer tweaks, but maybe you'll get better results with the full mod and it'll be more expensive if you want to undo the changes (original filters are quite expensive).