I concur to the notion that INFERNO just did not work! I tested it as well, try using a smaller range (below 8000 damage) to see if it makes a difference.
From what I've heard Damage Reduction is suposedly 0.3% reduction per point, meaning even at 1000 reduction you'd still get only as tiny as 300 reduce.
If we assume Injury works similar, then to see any kind of increase you'd need at least around 800-1000 of it, and against someone who has little to none. Enemy's injury counter-acts yours after all.
a simple test can be done on kakashi clone, unless kakashi clone changes his stats everytime we try him out on d same level.