those serving sizes are usually for wedding cakes which are double layer cakes......
i've done cakes for over 30 years and when you just serve sheet cakes you HAVE to increase the portion size.
If you add 1/3 c. flour and 1 t. vanilla to each cake mix you will have a better tasting cake.
And usually for things like this it's best to make vanilla cakes because not everyone likes chocolate! sometimes people ask for marble cakes.....if you do a marble cake the ratio to use of white to chocolate should be 3 to 1....if you do half and half you will end up with a mostly chocolate cake because the chocolate over powers the vanilla.
Good luck....I know it will turn out great!