use the poor man's dyno:
pick a flat stretch of hwy long enough that you reach top speed. go both ways and write down the mph, add them up and divide by 2.
then do whatever you do to your airbox and repeat the test. if your average speed is the same then there's no gain, if it's more or less then you can calculate the hp increase (or decrease).
to be relevant should be the same outside temp and relatively low wind.
when you're done take some pictures and post them along with your results