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.

Well its not exactly 'that' simple. To get into the 1000 range you'd need a gold or red damage reduction chakra in 8 gates, high group skill, and gold damage reduce in every ninja tool.