So in your testing the amsteel broke at the LB, or did it cause the splice to slip since it wasn't going straight into the center of the rope? Is this based on testing a closed loop? Is this the same for a loop at the end of a rope since the splice has to go through the braids rather than into the opposite end of the rope? Again, I'm just trying to educate myself here not doubting your word.
Also, 8-12 percent doesn't sound all that bad compared to the 20-40 percent one loses when tying a knot in a rope. On the other hand, it makes me cringe to see guys using 1/8" amsteel daisy chains to support platforms and sticks. I tend to err on the side of bigger is better though, so there is that.