My one pipe is a 9mm and I find the draw too open for me, but with the reducer to 3mm the draw is too restrictive. I've been considering boring the reducer out to 6mm. But I have learned to smoke easier on the more open draw.
Most things I don't care to use a filter. I haven't tried yet, but next time I smoke this Lane Very Cherry, might do so with a filter and that might make it better for me. That said, on the occasion I might use a filter, 9mm I think will be better than 6mm. At least with regards to charcoal filters. And I prefer the charcoal filters to balsa, as the balsa reduces the draw without reducing flavor, but I don't tend to have wetness issues so they're not doing anything for me (just reducing draw for no reason), whereas the charcoal filters mute the taste (hence why they might work out for me with heavily topped aromatics). Though, I do dislike how heavily restrictive the filters are. If they could make the same 9mm filter with 2/3 or 1/2 the amount of filtration and restriction, that would be cool.
Though, my experience with aromatics is limited and I haven't properly dried one yet before smoking.
I did just order another pipe today which is 6mm. Once I have that, I'll be able to contrast the 9mm with the 6mm to see how I feel about boring out the reducer.
So as it stands at the moment, what I like about 9mm is the 9mm charcoal filter. I can plausibly see myself having a few 9mm pipes with filters for rough blends and a bunch of 6mm for use without filters.
Only time will tell.