Which Tyres?

Not really. 25mm is not faster than 23mm. 25mm is neither more aerodynamic nor lighter. 25mm does not provide lower rolling resistance than 23mm.

Base on what? conventional wisdom of your heyday? there's already a decent number of test showing that there's marginal gain with 25mm tyres, even 28mm, the gain is obvious in the real world, as it's never as perfect as an indoor velodrome.

What it does provide is lower inflation levels for a similar rolling resistance.

The whole point of going bigger, to allow a lower inflations without damaging the wheels/bikes, less jarring on poor surface that result in better grip, more comfort to the rider, and most importantly, less fragile.

Back in the 1970s the tubular tyre of choice for Audax was the Clement Seta Campionato del Mondo. Wider tyres = Comfort?

Because in the 1970s, the conventional wisdom that skinnier tyres at a higher pressure = faster, and of course, the British audaxers tend to stick to skinny tyres at the time (the French does the opposite by going for balloon tyres).

Tubular is always more comfortable in the smaller sizes than their clincher counterpart if you insist on sticking to 23mm, especially at a high pressure.

Sure but lets not really talk about comfort when so many people here are riding low spoke count deep section carbon rims.

Wheels play a surprisingly little role in comfort (emphasised on comfort, not in term of aerodynamic for instance), the tyres itself make the biggest of difference in comfort, hell even the bicycles can be made comfortable by just choosing the right tyres for it.

You can be uncomfortable on a Mavic Open Pro wheelset with a 23mm Gatorskins at 100psi, and yet be very comfortable on a Zipp 404 with 25mm GP 4000s II at 75psi.

On the track where one can run very high pressures the narrow tyres are the top choice. They are clearly faster.

Because it's on the fucking track.