Max pressure for the .303 is given as 45,000 CUP or 49,000 PSI
If a 7.62X51 cartridge generates 52,000 CUP then it exceeds max pressure the Enfield rifle was designed to handle by 7,000 CUP.
If a .308 Winchester generates 62,000 PSI it exceeds the max pressure of the cartridge the Enfield was designed to handle by 13,000 PSI.
If a .308 Winchester is fired in a chamber proportioned for the 7.62 NATO it can have effectively .013 excess headspace, not wise with any high powered rifle.
Reloading Manuals give loads considered safe as possible to avoid accidents, they aren't authoritative on pressures of military ammunition under combat conditions fired in rifles that have seen decades of use and abuse, and which may have never been properly cleaned since leaving the factory 60-100 years ago.
I seriously doubt that the L42 Rifles were intended for use with the hottest 7.62X51 NATO available even then, much less the jacked up high performance ammo occasionally found on the surplus market, or ammunition degraded by several decades of storage under unknown conditions. The loads for the L42 would have been manufactured with accuracy in mind, and FN FAL ammunition of that time period was noted for its qualities.
I have a run down on barrel life of the Enfield rifling adapted to the FN FAL here. Extended barrel life was in large part attributed to the ammunition manufactured for rifle use at the time.
No manufacturer would try to sell .303 ammo loaded to 52,000 CUP or 62,000 PSI without a specific warning that it was not for use in Enfield Rifles. A P-14 might handle such loads if the rifle were in very good to excellent condition, but I doubt you'd find many who'd consider it wise even then.
No doubt if some owner of a Enfield converted to 7.62 has separated cases when firing .308 we will hear complaints about the quality of US cartridge brass rather than the differences in chamber dimensions.
That the .308 brass holds up in such an oversized chamber should be a testament to its quality.
