But, how much rolling resistance do you gain by the wider tire? Enough to offset the added aero drag?
No... That's hardly anything.
And note... the tests you site were of TUBULARS
. A 23mm tubular stays 23mm regardless of what rim it is on. A 23mm clincher goes to 25mm on a wide rim. If you want to replicate their 23mm tubular aero performance on your clincher wheelset, then you need to run ~20mm tires.
Yes, from a racing perspective, and likely at the higher pro level, they wouldn't run anything but tubulars...so expect the testing to follow, and the obvious benefits of tubulars.
No reason to run/test clinchers except for the convenience of fixing a flat on the road training. During a race, you got a wheel truck or short hop to the pits in a crit.
I simply went to the HongFu 40mm clincher for convenience/ease of flat fixing/tire swaps, and for now, I'm racing on them. They perform nearly as well as the 303 FC tubulars I just got rid of. Ride isn't as nice, or weight...but training on tubulars are a PITA if you flat to deal with on the road. Otherwise, yeah, test and ride tubulars...they are better overall.