I'm so happy to see a vibrant dialog about crank length!!!
It's one of my favorite topics.
There are obviously some strong beliefs about the topic as it relates to power output. Personally, I think that part of the discussion is little more than an academic exercise leading to nowhere.
Your power is not a function of crank length alone. And I know most people involved in this discussion are aware of this fact. However, the discussion of crank length is always dominated by comments about power output that lead the less informed to believe that the two are intimately linked (which they are not). Yes, there is a relationship. The impact of crank length is not nothing as it pertains to power output, it's just not everything.
Longer cranks absolutely offer more leverage through the power portion of the pedal stroke. This add'l leverage
can contribute to increased power output. It can also cause knee problems and hip impingement issues. How much power are you putting out relative to your FTP when you are uncomfortable and in pain? Whatever it is, it's not as much power as you would be putting out if you were more "comfortable" on the bike, which leads to my main point.
COMFORT = SPEED
Crank length is an important element of a bike fit. Your comfort on the bike and your efficiency / output is partly a function of crank length. I'm 5'6" with a 29.5" inseam and I have a functional leg length discrepancy. The 170mm cranks that come standard on 50cm - 52cm frames put added stress on my knees and do not allow my hips to remain more open throughout the pedal stroke. I guarantee I am faster for much longer with my 160mm cranks because I have less stress on my knees, my hip angle can open up, and as a result I am more comfortable.
And when I use the term "comfortable", I'm not talking about the type of comfort you feel lying on the couch. "Comfortable" for purposes of this discussion means "your ability to hold your position on the bike". For example, the "raciest", most aggressive setup on your beloved sub-6.8kg road bike with a 4" drop from handlebars to saddle will not make you fast if you can't hold the position. It's the same with crank length.
Lastly, I think context is very important as it pertains to crank length and which crank length might be "best" for you. If you are a pure sprinter you might want to compromise some comfort to be faster at the finish. Longer cranks can help with that. For just about everyone else you should be riding a crank length that is kind to your knees and to your hips. A low stack, long reach and long crank arms are not doing you any favors if the discomfort forces you up onto the hoods because you can't hold the position.