As Arizoni sez, You can't for more air though the filter. The best place for a filter to be is in a big box that supplies the volume of air that the filter and engine require without forcing the issue. If you try to force more in,as Arizoni sez, it just dams the flow. I'm not sure if the Enfield is the best setup as it comes, but without a flow meter and other measuring devices, dynos, etc, you'll never know.
I've seen one setup that used a K&N that is longer than the one you're showing on a much straighter tube running into the injector. It looks like a better setup, but I haven't seen dyno run data on it yet.
Bare