I haven't tried many cooks over 300 on my WSM with my IQ. When I have though I noticed in general that using just KBB makes it tough to keep temps above 300. And I run it dry as well. When I have done hotter cooks I have either mixed in some Royal Oak lump or run only RO.
The other solution and a pretty simple one was what you mentioned about opening additional vents more. Other may have other suggestions, but really it might just be that the WSM has trouble doing over 300 with the IQ....unless you give it a little help like opening more vents or adding some lump. Both I think are good solutions though because they work.
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