I'm pretty sure that snap shots are limited to four only. External 7/8 won't get you more snap shots. It will get you more stomp pedals... though six stomp buttons is *usually* enough considering a parameter can only be connected to one stomp.
However, an exceptionally awesome use of external 7/8 is to allow for a solo boost or other effect *while* you are using the six button looper! Since the looper uses all six of your switches, external 7/8 let's you have both.