Its pretty subjective from person to person and largely dependent on whether you have any experience running a shorty 5.56 or all you have ever used are rigs set up with longer barrels. Similar to LRRPF, I personally thought a 14.5" 5.56 with a 5.56 can (a suppressor that only added 5" in length and 16oz to the rifle) was ridiculous long for my intended uses where maneuverability and speed would be paramount. That said though, i was used to a 10.5" using the same can, so clearly the manuverability and speed aspects didnt seem anywhere near the same ballpark. To put it in perspective, i also use a .30 cal can that is 9.5" long - similar in length to your Specwar at 9" - on a 16" Grendel that I would never suggest it being the epitome of manuverability......and that can is 3/4 of a pound lighter than the Specwar.
IMHO, I would highly recommend a 10.3"-11.5" 5.56. They are awesome guns. Not the best for every application by any stretch, but awesome for the types of things youre planning on getting into. This particular scenario also highlights the benefits of a dedicated 5.56 can - specifically in the OAL and weight areas. A lot of the best 5.56 cans rated down to 10" are nearly half the length of your SS .30 cal cans and at least a half pound lighter.......Both are very conducive to the advantages you'll certainly want in the context of your application.
Looks like you get to buy another suppressor! ;-)