1 in 8 = one complete rotation every 8 inches. Faster RPMs = more stability for longer bullets. 1 in 12 = one complete rotation every 12 inches.
I have a 1 in 8 on my AR and it averages 3/4" at 100 yds with subsonic 240 and supersonic 125s. A faster twist will stabilize the shorter bullets sometimes to the point of over stabalization and bullet failure, but the Whisper is simply not capable of generating enough velocity to do this.