A while back I researched this quite a bit. Studies show very little difference in oxygen consumption between 140 to 180mm cranks. They show a marginal improvement with even shorter cranks. The difference between off the shelf sized cranks is probably not measurable.
A shorter crank though can be more comfortable with the wider hip angle and it allows one to go lower and be more aerodynamic.
Doing my own experiment I switched to 170s after riding 175s for decades. 170s felt good, especially during longer intervals on the flats. I felt like I could turn the pedals over faster and stay on top of the gear better. They promoted a higher cadence. I climbed just fine with them though I felt like I needed bigger cogs and a higher cadence. And just a 5mm change did feel more comfortable. I raised my saddle 5mm and kept my stem where it was.
I've seen a number of studies addressing this question and they all seem to have material shortcomings. For example,
1. Increasing the saddle height by the length of crank arm reduction. That increases diaphragm impingement slightly and could offset the improvements one might otherwise see in that respect.
2. Generally riders get to self-determine position after changing crank arm lengths. If a rider is riding upright, shorter cranks won't give the same improvements in hip angle and power output that might come about from being in a more aggressive position. In my experience fitting and coaching a lot of track riders, shorter cranks don't do much if the rider likes to sit upright, but the riders who pursue lower positions can achieve faster pursuit times. In short, this isn't a passive analysis with different crank arm lengths thrown in.
3. Rider fitness, experience, speed, and other parameters all affect the results. Riders who are reporting inconclusive results after riding 20-22 mph pacelines aren't necessarily a test of shorter cranks. The riders who are doing 35-38 mph in very fast massed start track events are testing the benefits of shorter cranks a lot more.
4. Hip angle may not be a measured outcome of shorter crank arm usage but rather a qualifier for it, with only riders with certain prior and post hip angles comprising the data set that correctly test whether shorter crank arms matter.
5. Even when studies are done on quote-unquote athletes, very few are being done on qualified racing samples. The Australian and British track cycling teams have done extensive studies on their own riders and most riders have moved to shorter cranks based on performance data. I was told Wiggins is now on 165 mm cranks at his last major track appearance after years of riding 170 mm minimum. He also tested 165's extensively for his hour attempt; I've heard varying reports about whether he rode them or not (the press coverage was based mostly on test bikes and those varied between different crank lengths and other parameters so the reports may not have been accurate).