I have two AR15 pistols with SB15 braces as well as several AR15 rifles with full stocks, and an AR15 SBR.
I personally can't tell the difference between shooting a full-stocked AR15 and one with an SB15.
I'd be lying if I said I didn't prefer the SBR to the SB15-equipped pistol. The SB15 is kind of bulky and inelegant looking, but functions just fine from the shoulder. My only real complaint about it is it can't be adjusted for length of pull (ok, it can, but not without modifying your tube).
We see SB15 equipped pistols all the time in our Defensive Rifle classes, they are very popular and have some advantages over NFA.