The +10% rending sub-talent for the Marksman Focus keystone doesn’t appear to add any extra rending at all, at least in the Psykanium. I’ve tried it with 10 stacks and 15 stacks, and both granted no additional damage to any kind of armor type with the rending talent equipped as compared to unequipped.
Steps to reproduce: build all the way down to the Marksman Focus keystone, and get yourself 15 stacks. Then shoot whatever you want in the chest or head. Next, put a point in the +rending talent, and repeat the same steps as above. Compare damage numbers.