The only verifiable difference I observed with my bikes (mix of 165 mm-175 mm crank arms) was that the longer crank arms required more effort to spin at a high cadence. I did switch the crank arm length on my then dedicated distance roadie from 170 mm to 175 mm, when I switched to a triple due to a nagging knee injury. The idea being that my torque sensitive knee could use the extra length to keep torque as high as possible. I don't really know if the extra length helped, but it didn't hurt in my situation.

Unless I rode two bikes with different crank arm lengths back-to-back, I couldn't feel any difference. The feeling went away rather quickly, however.

Brad