I get this question a lot, having written Brining 101.
I think a lot of people guess that it would be bad to do, but haven't tested it.
For me, I don't like to pay someone else to brine, so I try to find them non enhanced. However, I've tested with up to 12% and found no real effect.
Too salty? Can't see this happening. Science is on your side. Osmosis means that the bird will try to achieve equilibrium with the brine solution. If it's already "salty" then it won't take on any of the brine solution. In some cases if the brine outside has less salt, then it will tend to draw out salt from the bird.
the challenge in this is what did they inject with and what's the salt % of that solution.
Turkeys are cheap enough and I say give it a try. Use a normal brine solution, and I like to go 36 to 48 hours. We never get a complaint of salty birds around here.
More details in Brining 101 if that will help, but you're question is being addressed in Brining 202 (hope to get this out this year before TDay).