What sort of engine is it?
Ive been thinking about doing the same, and adding a pod filter instead of the panel filter. I dont know the exact values of gain in power, but AFAIK the air your engine inducts should be as cold as possible -
So try this: Go for a drive, then stop and open your bonnet and touch your air box - it sure as hell aint cold.
Ive heard you can design these things yourself - ive even seen certain setups where a bag of ice can be added