This kind of reminds me of those "Mikey" commercials. No one wants to try it until Mikey likes it. I bet "Mikey S." is going to like his Ninja.
What's the likelihood that Team Mugen isn't going to make this engine work for them? Slim to none. GRP is putting a lot on the line by partnering up with Mugen Seiki. When Team Mugen wins several races with the Ninja, everyone can talk all the dirt they want - but the bottom line will be that the engine is a proven winner. But if they can't win (unlikely), it will just become another engine to choose from in the LHS showcase.
Team drivers are team drivers for a reason: They know how to tune and drive anything you hand to them. If they can't win with their own company's products, then it's unlikely that consumers will believe they can win with the company's products.
How much does or doesn't a factory team's success (or lack thereof) factor into your buying decisions?