I agree with the consensus of this thread.
Putting higher quality shrimp in can either improve the quality of your batch or over time degrade the quality of your additions, in the end it is just the amount of genes averaging out, if you have more low quality shrimp, than the average will drop down, and vice versa.
That is the whole reason shrimp keepers practice selective breeding, or culling. If you remove the lower quality traits from the gene pool, you enhance the likelihood of your offspring having more desirable traits.
You really are working against yourself by not culling the worst looking ones. I want to note here that culling is not killing, culling is merely removing the genes from the breeding pool. I keep all my culls in a separate tank, because there is still a miniscule chance that a cull's offspring can have a nice coloration, in which that shrimp will be allowed back into the main population.