As a rule, a CTE will NEVER improve performance. A CTE is essentially a disposable view. There are no additional statistics stored, no indexes, etc. It functions as a shorthand for a subquery. In my opinion they can be EASILY overused (I see a lot of overuse in code in my job).
Check the cupcakes at the minimum baking time. If the recommended baking time is 18-22 minutes, check them at 17 or 18 minutes (just a quick peek, make sure not to let too much heat out of the oven). If the tops are rounded and no long appear wet, they may be done.
Cupcakes are done when you can insert a toothpick into the middle of one and it comes out clean and dry. Always wear oven mitts when removing cupcake pans from the oven. Place cupcake pan on cooling rack for at least 5 minutes. Remove cupcakes from the pan and place them back on the rack to cool further. Let the cupcakes cool completely before you decorate them.