My client is not sure about the impact a particular product design would have, but insists that the product design that he suggests is workable. The Project Managers know there are other options that will work better.
What actions can we take, and how can the team handle this issue strategically?
The Project Managers know there are other options that will work better.
The implication behind this sentence is that the PMs know with absolute certainty the other options will work better. If my inference is right, I find that implication odd as I am not sure I have ever experienced a time when a complex problem had a solution that was guaranteed to work better than another solution. Guarantees are best left with the sales and marketing people.
Every solution to a complex problem has its fair share of benefits, costs, risks, and penalties tied to it. If you think there is a better solution than your client's, build and present the case and an non-emotional, objective way. Present your findings for all potential alternatives on all four of those things. Give you recommendation and let your client make a choice. Then implement his choice with the professionalism he needs.
EDIT: How to convince? I think it becomes a sales call and, as with any product you are trying to sell, it becomes an argument that shows how the benefits of one product over another fulfills a client want or need. You need to be able to substantiate that the features, advantages, and benefits of your recommendation meet your client's needs and wants better than the other alternatives and that the costs, risks, and penalties of that recommendation are appropriate and less than the material benefits. I can't break this down into something more tactical without knowing more of the specifics.
It is less about you convincing the client and more about the client realizing a better choice. So the truly best alternative should sort of pop out in your business case.
Treat it as a change request. Assess the impact on scope, schedule, cost, quality, and conformance with mission needs. Find Key Performance indicators to discriminate between the options. Make sure that the stakeholders are aware. Then decide if the impact on the project is sufficient for you to walk away.
Be careful; As @David Espina hints, their preference may reveal that they haven't correctly documented the scope/requirements. If they're insistent on something, analysis may reveal that what they're insisting on may actually be a critical requirement that they haven't articulated.
If the client decides to go with the suboptimal design, and you choose not to walk away, then this is a risk management problem. How can you mitigate the risk? Timebox? Ask for a prototype & evaluation? Ask to develop what you perceive as the optimal solution as a backup in case the primary solution fails to achieve quality thresholds.
The Project Managers know there are other options that will work better.
Firstly, the pursuit of optimal solutions is not always necessary. A project succeeds if it is "good enough" for some defined purpose; unless the client's solution will cause the project to fail, it's probably not useful to press the issue if the client has already rejected well-presented alternatives.
Secondly, the underlying issue here is really about project risk. If the client's solution puts the project at risk, or is unlikely to be happy with the deliverables even if they are exactly to specification, then your PM-fu is best spent on transferring risk to the client.
This could mean any number of things, depending on the circumstances:
There are certainly other ways to mitigate or transfer risk, but it's at least a place to start. If you've done your professional due diligence and presented the risks to the stakeholders, the success or failure of the project is ultimately their responsibility.
