I'm doing a hot and fast brisket on my 18.5 wsm. I calibrated my Maverick and Thermapen last night with ice and boing water. They wer "spot on". When I fired up the smoker and it hit 300* on the Maverick, the WSM showed 225*. If you're using the bimetal thermo from the WSM to determine smoker temp, you're probably running the WSM at a much higher temp than you think causing faster cook times. Just a thought.
WSM 18.5", Weber Kettle, Thermapen, Maverick ET-732