What is the measured difference in performance on race 2cv’s fitting a straight 24/21 instead of 26/18?

Viking,
as there's barely 1% difference in total choke area between the two types, you'd be hard pushed to measure any meaningful difference.

Without any permitted modifications, the 21/24 is less susceptible to fuel surge/starvation on fast cornering, but that can be addressed by altering the position of the drilling connecting the two float chambers on the 18/26 type.

ken.

p.s. Much more likely to notice a difference by making sure that 'full throttle' actually has both butterflies in the wide open position.
I've lost count of the number of carbs I've found on road-going 2CVs where the butterflies go to 110% open, which is exactly the same as 90% open...
Moral of the story, don't use a throttle spring which could double as a bed spring and also ensure that the clip on the throttle cable adjustment is set so that the spring at the bulkhead end is 'only just' compressed at full throttle.