Yesterday was a range day,

so some shooting was done before grocery shopping and such needed doing.*  And the shooting included an interesting result of the 7.62x39 subsonics.

REMEMBER THE DISCLAIMER: Neither of these were published loads, they were worked up by me and there's no guarantee they'll work or be safe in your guns.  So keep that in mind if you try them.
Wind was suitable so I gave them a try at 100 yards.  First the 11.7 grains A5744 under the 255-grain Sub-X bullet

First aiming point was the upper left diamond, and they hit in a decent group about ten inches down and about four inches right.  The scope is an old Leupold Pig Plex 1-4x; it has a ring around the center of the crosshairs, and a stadia line below and on each side of the ring.  Second group I used the stadia line below to aim at the diamond, which put the group nicely almost even vertically with the aiming point.  One thing you'll notice of you look close: those holes aren't quite round.  I'm guessing that the rifling pitch isn't quite enough to fully stabilize this long a bullet, and at 100 yards they're starting to cant just a bit.

Second target was the 11.9 grains load:

First shot was crosshairs on the upper left, which hit about the same point as the 11.7 load, so the second I used the stadia line on the same aiming point to fire two, one of which I pulled low.  The next three used the stadia to aim on the center diamond.  You'll notice two things: first, the 0.2 grains more powder gave no elevation increase from the ~60feet per second gain; second, one of the last three seems well on its way to keyholing.

My verdict is stick with the 11.7 load.  And 100 yards would be the max effective range, as long as you can compensate for both drop and right drift.