On my track bike, I changed to the 34 cm Scatto 3T bars. A couple of years ago, the Brits claimed they were .1 seconds faster in the flying 200 meter. I did a 2 hour structured track session a couple of weeks ago and the total mileage was 34 miles going around the 250 meter track. Most trackies have gone to narrower bars.
I have broad shoulders. On my road bike, I have a 44 cm bar. I tried the 42 cm but the wider one seems more comfortable. My posture goal is to have my shoulders down and relaxed and maintain a lighter touch on the handlebars keeping my spine straighter and the vertebrae better aligned. Over time, I have improved my hip flexibility and substantially improved my core strength and endurance. As such, I have lowered my handlebars and increased the stem length. I find this position more comfortable.