You must be careful, for a project is a dynamic thing.
In a few words, things are going to change, no way; if you don’t adapt the project to the situation, the project is going to fail.
That is the reason why I, as a PRINCE2 Practitioner, keep in contact with the customer more often than foreseen by the book (Project Management methodology), i.e., I sometimes contact the customer even if we are not at stage’s end or going to go beyond tolerances.
Under these circumstances, how do you define a “performance bonus”?
Unless you want to produce a lengthy legal document, it is up to the customer to decide whether the bonus is due or not.
In the event, I prefer to do this: the customer is going to pay 80% of the due value; she/he will pay the remaining 20% if the project exceeds expectations. Call it discount, call it incentive, the customer is happy and I usually get the 20% bonus, for a happy customer very rarely doesn’t recognize due merits (i.e., greater than expected benefits for him).