An increase in false negatives would be of greatest concern when reviewing the performance of an organization's IDSs, because it means that the IDSs are failing to detect and alert on actual attacks that are occurring on the network. False negatives can lead to serious security breaches, data loss, reputational damage, and legal liabilities for the organization. False positives, on the other hand, are alerts that are triggered by benign or normal activities that are mistaken for attacks. False positives can cause annoyance, inefficiency, and desensitization, but they do not pose a direct threat to the security of the network. Therefore, a decrease in false positives would be desirable, and an increase in false positives would be less concerning than an increase in false negatives.
References = CISM Review Manual, 16th Edition, page 2231; Intrusion Detection Systems | NIST