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.
PREHEAT oven to 350Â°F for metal or glass pan(s), 325Â°F for dark or coated pans*. GREASE sides and bottom of pan(s) and flour lightly. Use baking cups for cupcakes. BLEND cake mix, water, oil and eggs in large bowl at low speed until moistened (about 30 seconds). BEAT at medium speed for 2 minutes. POUR batter in pan(s) and bake immediately. BAKE in center oven at 350Â°F following chart below. *Add 3-5 minutes to bake time for dark or coated pan(s).