The LT500 loves to breath, we tested with a 39 air striker and 6" K&N and went with 41 mm and went with a 9" K&N and picked up over 3HP over all using Rouges Air Filter Bracket

Without air flow and fuel flow data. You really cant say 100% the air filter was the gain for 3hp.
With a filter switch you likely see air flow bump, and BSFC drop. If you take that BSFC with the 3hp gain and use it as a target spec. Then go back to the 6" filter and drop the BSFC (lean it) to the same spec as the 9". You will likely see that same hp gain