This illustration was unfortunately created by someone who doesn't understand that the pressure levels quoted are in 2 different systems of measure.
Enforcers/L42s ect are not MAG58s - sniping and target rifles are not headspaced at the high end of the chamber spectrum - what would the point be of a sniping or target rifle with excess headspace or an extra long throat?
.303 Britishand .308Win nominally have the same H2O case capacity (56grains of water).
.303 British 174gr MkVIIz was loaded to 2440+/-40fps
.303 British 175gr MkVIIIz was loaded to 2550+/-40 fps
MkVIII was cleared for use in rifles
Federal GM Match 175gr is loaded to a nominal 2600fps which is at the high end of the MkVIIIz velocity window.
There is no "free lunch" - 2 projectiles of the same weight loaded to the same velocity in the same case capacity are not magically dramatically higher in pressure.
I know a number of people who are running Federal GMM 168gr in their L42s and Enforcers.
personally I am running 165gr Hornady interlocks over 42gr of IMR4895... safe in my rifle ect ect.