Function points are not a very good measure when sizing maintenance efforts (fixing problems) or when trying to understand performance issues. Much of the effort associated with fixing problems (production fixes) is due to trying to resolve and understand the problem (detective work). Another inherent problem with measuring maintenance work is that much of maintenance programming is done by one or two individuals. Individual skill sets become a major factor when measuring this type of work. The productivity of individual maintenance programmers can vary as much as 1,000 percent.
Performance tuning may or may not have anything to do with functionality. Performance tuning is more a result of trying to understand application throughput and processing time. There are better metrics to utilize when measuring this type of work.