It's just my opinion, but I don't think we are making the kind of HP needed for some of BIG filters we put on.
If you crunch the numbers, sub 9000rpm, sub 90hp and a carb thats a tad over 40mm. We are over kill at 6".
To do a dyno test to really show if they like / need that size filter. You need air flow and fuel flow data. Along with two new filters, not new vs used. When you use A/F as your guide, it's after the event and a fuzzy gauge at best. With air flow you should see the gain (if it needed more area). Then your fuel flow vs power would give you BSFC. From that you go back to the 6 and verify with the same BSFC that the power was increased with the 9. That would be apples to apples.