Did a simple test (3 maps) on dyno a few years back - (Map 1)Used drop in filter (Blue Print) as the base map, (Map 2) new HKS hybrid high flow filter (2 BHP gain), (Map 3) filter completely removed (another 1 BHP from map 2). Assumptions made: All 3 runs are on the same condition and dyno is mapping consistently throughout the test. As an engineer, i would at least test each condition twice to ensure repeatability =) i actually only paid for 1 run but the guys there were interested to see the difference too hence gave me 2 free runs.
Since then i just put any drop in filter as long as they filter the air going into my engine. Either Blue Print or Mann filters, around £8 each per year