Yes, I would be concerned. It isn't as bad as other designs I have seen, but I would make sure the client understands your concerns. Also, make sure the clients attorney is also on board unless you are prepared to defend the formula yourself.
A few years ago a lot of people were basing cash balance credits on a "special bonus". The IRS accepted this for a few years, then they stopped accepting it and started to challenge it. They even went as far as revoking previously issued determination letters.
If you have a determination letter for the plan you are in a much stronger position, but even then, I would be a little concerned.
Also, just because I would be concerned doesn't mean it isn't accepted in certain circles and hundreds of people are doing it.