Before you go changing cranks, you ought to make sure your saddle height is correct and that the fore/aft is properly set. The last set of cranks a 5'11" person with a normal inseam should be on is a 160mm. That's plain dumb.
If you shave 12-15mm off the crank and leave your saddle where it is, you're going to have a completely different set of problems. Sure, you don't have to raise your knee as high, but a shorter crank will also compress your knee at the bottom of the stroke.
It's a bad idea. The problems you're describing sound far more like a poorly positioned saddle than something to do with "crank force." I have a complete POS right knee and I would never consider riding anything else other than a standard 172 or 175. A 160 is for tiny folk.