I'd go for the panel filter, and look at the size of the inlet port to the airbox. It could do with opening out. Cover the airbox in reflective insulating material to stop heat absorbsion.
Thanks, I have had two Imprezas, three TVR's and a Clio Williams over the years and always fitted K&N filters to them as standard, I found they all just picked up that bit better on the throttle response..