I did a butt on the kettle a while back, just to try it.
Did a Minion start, lighting only 10 briquettes, behind a couple of fire bricks. Even with the bottom vents closed, the temp climbed to well over 300.
You always see folks say of the WSM to keep the top vent wide open. I had to go against that for the OTG kettle to get the temp down. It worked fine.
My neighbor cooks on three Weber kettles, and he often closes the top vent to just a crack, and his food comes out great.
How has your experience been with temps on the kettle?