My cakes hump in the middle despite the use of the cake strips. I end up cutting off about  a full cake mix to get it flat. Any suggestions? Also is there any way to gain volume from a box mix?

I bake mine at 300* rather than the usually suggested 350*.  It does require a longer baking time, but baking at a lower temp allows the sides to bake slower and rise as high as the middle.  If there is a slight hump when I take it out of the oven, I use a clean dish towel and gently press it down while the cake is still hot.

I had this problem too even while using the cake strips... Then I realized I wasn't using the strips correctly. Now this may not be your case, but I'll give you my piece of pie on the subject..

I realized that you really need to soak the stirps, not just wet them. When you run the strips under running water on the same spot, you'll notice that it turns a gray-ish color. That area is now soaked thoroughly. Do this to the entire strip, then remove as much of the water from it like you would normally. I've done this ever since and I get only flat cakes. If I do get a hump, it's only a slight 1/16 inch of a hump. 

Also, knowing your oven helps. If your cakes ends up overbaked when you bake at 350°, then definitely try baking at 325° next time. I bake my 6 and 8in cakes at 350°, 10in cakes at 325°, and 12 and 14in at 300°. I only go up to 14in because that's the biggest size tier I can fit in it ;D

Having to cut off a full cake mix worth when you level your cake could possibly also be that you are using too much batter in your pan.  

Are you making any changes to what your cake mix calls for?  

Is this an issue with all pan sizes and shapes? Sometimes the true square or true rectangular pans seem to create this issue .

Have you tried resting your battered filled pans on the counter for 20-30 minutes before baking?  I  do this with cupcakes and cakes to get a nicely slightly rounded cake top which is easier to level when you need to.  It works well with mixes and from scratch although I wouldn't recommend it for recipes that rely on eggs as the main leavening agent like chiffon or angel food type cakes.  My kitchen is always at 70 f so there are no food safety issues. 

I have found out that if your cake batter is around 70 to 73 degrees you will have a nice top to your cake.  If you aren't sure you can stick a thermometer in to test the temperature.  If too warm, just set your batter in the refrigerator for a bit to cool down.  :)