I went from 30 mm wide 451 (20" x 1 3/8") on my Bike Friday to 650B x 42 mm. Going from 65 psi on the narrower tires to 42 psi on the wider ones was a pretty significant gain in ride comfort.
In the first year I didn't actually see much evidence of a difference in speed so I concluded it was about the engine after all. I did feel fresher when I arrived at the brevet goal though. In the next two seasons my brevet times improved significantly though I'm not getting any younger and had actually put on weight.If anything, that big front bag for the porteur rack encourages also me to bring more stuff.
I feel much safer on the wider tires. Ultimately, maintaining my "Century a Month" status (at 85+ months now) is all about avoiding injury. Gravel that would have worried me on the narrower tires I don't even think about and awful roads that would have been scary before I will still ride with confidence.
On the narrower tires I used to think 2,000 km (about 3 months) between punctures was pretty good. On the BSP and BSP EL I've averaged 14 months and counting so far (3x in 43+ months). My longest puncture free stretch on the BSP lasted for 22 months.
I'm sold on 650B. Given that the vast majority of my riding is on roads I probably wouldn't benefit much from going from 42 mm to 54 mm. I don't know how much comfort I'd give up if I went to 38 mm, but given that my Elephant NFE has enough clearance for 42 mm with fenders, there's no real reason for going for anything more narrow but with slightly higher pressure.