Most ES won't have been stressed to the max. The majority of ES parts are used for functional and compatibility testing. Very few ES samples are used outside of say Intel for things like thermal stress testing or accelerated aging testing.
As far as materials, they should be exactly the same as production parts unless there was a defect discovered in the manufacturing or assembly process which is extremely rare. The dies themselves are run on exactly the same process and the substrates and associated parts are exactly the same as for release as they are just early parts. It would make little to no sense to use different materials since the whole point of ES is to test for volume production at external customers (OEMs). Yes there are internal ES part at Intel, but those generally don't even have labels and rarely make it outside of projects.
