Personally, I think with 7 inches supported with 8 inches overhang, you should be OK without support.

There is a seam at the 7 ft mark where it connects to another piece. That seam split yesterday (less than one week after install) which raised our concern. Come to find out the epoxy was only half way through the seam. I believe we will install corbels for piece of mind (now that the epoxy has been reapplied) but am disappointed for not knowing we could have installed the hidden steel versions. I own that but think a contractor might have offered that upfront given he did think it was a potential issue. Thank you for your feedback.

Here is a really helpful website that help you decide what type of supports to add.

http://www.granite-countertop-info.com/granite-countertop-support-brackets.html

With what has already happened..I would do the corbels for sure.