Dig - keep in mind that chamber length is not the definer of loads due to differences in standards between CIP and SAAMI. As stated, a 2 3/4" chamber does not make a gun proofed at 2 3/4" chamber, 1 1/4oz shot, and 3 1/4 tons suitable for 11,500 psi SAAMI spec shells.
Longer hulls perform satisfactorily if, IF, the service pressure the gun was design for is respected in the loads. CAUTION - guns with matching chamber length and shell length may not perform satisfactorily if the service pressure the gun was designed for is less than the pressure of the loads used.
HOS - yes, it is possible that an individual gun may prove satisfactory for higher service pressure than stated on its proof stamp. However, why take a chance of knocking it off face when there is an abundance of modern guns known to be suitable for modern high pressure ammo? Especially, why risk an expensive and historical gun? Even as an engineer with a background in design and strength of materials, I would not make "eyeball" estimates of action strength. The performance of low pressure loads is entirely satisfactory and they are easy and economical to obtain or reload.
The generally handmade guns from the UK (the vast bulk being made before 1960) are a finite resource. They will someday be used up and there will be very few more. As this generations custodians, we have a duty to following generations to use them carefully. My gun safe is not a museum; it is a rack of working guns. However, each will be carefully fed and cared for. In so doing, "someday" will be kept far in the future.
My hat is off to Dig for some of the orphands he has rescued and returned to service. I firmly believe that "returned to service" is the difference between a dubious decoration that soon goes to the scrap heap and a working artifact left to the care of future generations.
