Personally, I think there has to be some sort of vee across the tail. Single concaves are not meant for guns. The board you saw may have been a big small wave board?
My eight footer has a bit of concave (like maybe two mm) of concave in the belly before running to a panel vee through the tail.
With the 9'6" I have a little bit of nose vee - pretty common in big guns to part the chop - into a flat into a slightly rolled vee instead of the panel vee through the tail.
My 7'2" gun has a single concave nose, into flat, into a vee with a double concave through it.
There are lots of different bottoms that work, but reputable, experienced shapers are a must for me when I order a gun. Vee is a tried and proven bottom in all sorts of big wave big boards.
I have two more on the way too.