Mystery shopper programs are not just for large operators, although they use them regularly. It's useful for every cafe or restaurant to have an outside set of eyes to check service, food & beverage standards and all the elements that create atmosphere.
A mystery shopper will be checking staff behaviour, menu consistency and the many small details that set you apart from the competition. With repeat visits, you can use the feedback to keep driving improvements - this is objective information, not just opinions from friends of the boss!
You are also creating measures to justify rewards and incentives for top performers, or it could be disciplinary measures if there are repeated shortfalls.
Measuring and monitoring service is the first step to improving it, which leads to increased sales. You need an unbiased, anonymous assessor rather than in-house representatives that will skew the results, and a consistent program so the results can have a multi-layered effect.