Let's take the argument that the silver is caused by a weim cross. So what?
If we look at the Dalmatian/pointer backcross project that sought to correct uric acid production in Dalmatians, it was generally accepted that by the seventh generation after outcrossing, the dogs were in effect purebred Dalmatians again -- and healthier than before.
So, if weims were crossed in at some point (not saying they were), big deal...at what point would they be considered purebred Labs again?
If you say 'never,' regardless of number of generations, phenotype, temperament, et al., you're ignoring the evolution of purebred dogs wholesale.