Built in thermos tend to be somewhat sketchy regarding accuracy. I'd suggest temping at grate level with a good temperature gauge for consistent accuracy.
WSM's are great products and we use them to compete. If the $399 WSM price tag is a bit steep, consider the Pro Q products. There are a few left here in AZ at a great price. The comparable Pro Q model to the WSM 22.5 is the Excel20. Look on Craigslist or in the BBQ classified section here at AZBarbeque.
There are a lot of Pro Q user's here on the forum, and there is even a separate discussion category just for the Pro Q product.