My question is does any one have any type of signoff exhibit/language they they send with completed tests that requests that the client review the data and/or the tests and/or the plan specs and sign off and send back for their records?

While we ask clients to certify the data provided to us for completing the testing, we do not ask for any type of post-certification. That seems to me like asking the client to certify we did our work correctly.

We ask the client to sign indicating that they have actually deposited the contribution that was calculated and used in the testing.

not for testing, but for cross-testing plans I do try to send the contribution breakdown (by class or participant) with a space for a signature at the bottom. years ago the IRS 'implied' this was required, though I think they backed off a little on the issue. but once you get in a habit of doing something, its hard to stop, and it can't hurt.