I took out these 2 plastic things from the bottom of my stock 2004 cbr1000rr air filters, other than "maybe" prevent the filters from rotating a bit, I didn't see any use for them, I also noticed the main round part goes into the filter over an inch blocking a decent amount of the element from letting air through, I read somewhere that these plastic things where made to comply with someEPA or euro-3 crap, but please post if you think they serve another purpose. Or if my bike it's going to blow up. The ride feels better but it could be my imagination, well take a look at these things:

 

I have read the same thing about them being some sort of noise reducer. When I installed a set of BMC race filters I also left these things out, and do not see any way it will damage your engine. I do not notice any added intake noise with the new filters and these plastic rings removed...but my exhaust is so damn loud it probably covers it up. Now I need to work on a better exhaust system!

 

Well, the factory put them there for a reason - maybe your dealer can explain if he's knowledgeable (good luck with that) or perhaps get ahold of Honda and they may explain it (they may tell you to ask your dealer!). If you have an extended warranty, you might want to leave them in.

 

sound reduction
if they were important for peformance aftermarket filters would have them