Can a loadcenter main breaker be greater than the busbar ampacity?
The NEC requirement for protection of the panel is straightforward:   


  408.36 Overcurrent Protection. In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supplyside of the panelboard.  


So the panel must be protected by an OCPD at or below it's rating, but that OCPD can be the main or upstream.  As long as the upstream OCPD satisfies this rule, the main breaker does not have to;  a main breaker larger than the panel rating doesn't satisfy the requirement and doesn't protect the panel bus but it's not a violation to have it there serving as a switch / disconnect.  

Keep in mind that the NEC also requires that the panel and the main breaker be listed, and used in accordance with it's listing and the manufacturer's instructions.  The combination of a main breaker and panel with a lower rating may violate the manufacturer's instructions.  That of course would vary case by case.