I ride 180mm cranks and I dont think I'm going anywhere faster. These do feel more natural for my knee flexion and feel nice for constant, but slower, cadence. With 175s spinning up and going 100+rpm feels a lot easier, these go on my "go fast" bike. It's all preference and adjusting a bike for its intended purpose.

You might hate 180 cranks though. I know people that claim they make their knees hurt.