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.
Line 24 cupcake tins with paper liners. In a large bowl, combine cake mix and Sprite together. Continue to mix until the batter is smooth without any lumps. Pour approximately 1/4 cup of batter into every cupcake wrapper. Bake at 350 degrees for 20 minutes.