The D&B C system is one of my favorite conventional boxes still, but Tony is quite right, the C7 was never designed to be deployed in multiples. If you're looking for a sizeable rig as you've mentioned, you'll want to stick with the C4 as the C7s pattern doesn't lend itself to being in an array with 6 per side (the quantities you mentioned). I'm currently using two C7 tops over two C7 subs per side as sidefill on my tour. I had previously used a C7 top and C7 sub over a B2, but need move coverage on stage this time around. I'm regularly impressed at how loud these boxes get and how good they sound.
When you're back at your drawing board, I highly recommend seeing what you can do in C4 land, including looking at B2s. The B2 sub is still an amazing box and one of my favorite subs out there!