One doesn't need to SBR to have advantages over a common rifle caliber carbine.If I'm reading this correctly, you want something larger than a pistol but smaller than a rifle for defense in places where concealment isn't a primary issue. In that role, I don't have a problem with 9mm. As others have said, it does pick up a little velocity. As barrel length increases, 9mm can start behaving more like .357 Sig does from a pistol and .40 S&W can start behaving more like 10mm does from a pistol. You also get the benefit of light recoil and it is less loud than .223 or more powerful rifle rounds.
The question I'd ask is if you can't get by with a carbine. Versus a carbine, how much advantage do you get out of the "bigger than a pistol but smaller than a rifle without sending you through the unconstitutional hassle of obtaining permission for an SBR" class of firearm?
A loaded Beretta CX4 Storm weighs about 1.5 pounds less than a typical loaded M4 style AR, and is the same OAL as the collapsed AR. That 1.5 pounds is a huge advantage for many shooters.
The CX4 also has far less recoil, blast, db, and flash, all huge advantages (especially indoors).
And finally, no rifle caliber carbine can touch the 'practice per dollar' advantage of a 9mm PCC. On average it's about a two to one advantage.
As a criminal I would be far more worried about a confidant soccer-mom type that practices with a CX4 every month....
Than a tuff-guy John Wick wannabe that spends more money on tacti-crap than practice, and thinks 556 will somehow compensate.