how much does a rear end devalue a car

Usually the amount shown on the repair estimate less then deductible, unless the insurance company decides it is a total loss. As this is a Tesla it is highly possible it could be.

Devaluation is an economic thing, and it depends on the size of the wreck. If it's small, and properly fixed, likely no devaluation at all. If it's substantial, and fixed properly, it will likely have some noticeable impact. If substantial and not properly fixed, and it affects handling, get it fixed right.... I see an attorney in your future.

Its best to go by what the insurance company says. they're on the hook for this.