Hi olly
I've no experience of BMC filters but have used K&N filters on other cars. In general, airflow through the standard filter is more than adequate so your unlikely to see any performance or economy improvements. On the plus side they tend to last a lot longer so can work out cheaper long term.
If your changing the A2 filter yourself, note the car needs to be on ramps or over a pit as there is not enough clearance to remove the filter from under the front bumper.
Cheers Spike