I've tried the cake mix extender to my cake mix and it's not high enough. I want a tall cake with volume using my 13 x 9 cake pan.

This is what I used..

One cake mix and all the ingredients it called for + an instant pudding,
I also used:

1 c. of flour
1 c. of sour cream
1 c. of sugar
1 egg

Has anyone used this plus any baking powder or baking soda?

I know there's another cake mix extender that doe NOT have sour cream, but it does have baking powder. Any advice?

Switch to self rising flour and see if you like that better. Now I don't add the pudding though so...but I like mine with the self rising.

According to Wilton, a 13x9x2 pan requires 7 cups of batter. A 13x9x3 pan requires 11-1/2 cups of batter.

Wilton 2 & 3" party cake preparation and servings charts:

http://www.wilton.com/cakes/making-cakes/baking-party-cake-2-inch-pans.cfm

http://www.wilton.com/cakes/making-cakes/baking-party-cake-3-inch-pans.cfm

A half recipe of the WASC cake (using DH white cake mix) yields a tad over 7 cups of batter. (A full recipes makes a tad over 14 cups of batter.)

Expanded flavors version of WASC cake:

http://tinyurl.com/2cu8s4

If you wish to overfill your pans, you'll need to make another batch of your recipe...

Using a box of instant pudding in addition to all the ingredients you listed might lead to your cake falling upon cooling (too much sugar).

Handy cake trouble shooting charts:

http://tinyurl.com/2p5bdu

http://tinyurl.com/32goqe

http://tinyurl.com/6c745g

HTH

Then dble the recipe, and put more batter in the pan.

Mike