The Proof of the Pudding is in the Eating

This commonly used phrase (or more commonly used shorthand version: the proof is in the pudding), perfectly summates performance predictors.

The phrase means to demonstrate something to be true. Or defined in marketing terms, the true value or quality of something can only be judged when it's put to use.

In the case of performance predictors, I would argue that the prospective customer does not need to actually experience or use the product or service to make a purchase decision (or the implication would be that every business in the world would have to provide 'pre-purchase trials' on all products).

Powerful performance predictors will remove the pre-purchase doubt and result in prospective customers feeling confident in the ability of the business to deliver on its promise.

The Proof of the Pudding is in the Performance Predictors.