I'd bet that an IQ110 in Max Blow mode would do it. [ame="https://www.youtube.com/watch?v=bhcvfhm2Mdw"]IQ110 "Max Blow" Feature for Air Hungry BBQ Pits - YouTube[/ame]
Generally, when I need to go over 325 in my 22.5", I just run it without a blower and set the lid slightly ajar. That allows for the exhaust needed to get to higher temps without cutting an extra vent in the top. The reason the WSMs have trouble reaching higher temperature is the limited exhaust. There's plenty of intake to make it happen.
On my 18.5" getting to 375 is not a problem, even without any mods or door/lid propping. The Guru or IQ110 should be able to maintain it pretty easily.