Requirements are nice but an APM initiative need not begin at the start of a new application life-cycle. Reality is that a menu of apps are available; some mature, some problematic, some of little consequence – and the very few ready to begin with a ‘greenfield’ of clean requirements. You need only assess what proportions are on the ‘menu’ and you can devise a program that will bring them all into good alignment.
Most important is to actually begin with the ‘stable’ applications so that you can practice your deployment and configuration of APM and learn what your stakeholders really want for performance metrics. Then move on to problematic and ‘greenfield’ apps.
Every client environment is different and simply taking an academic and rational approach to performance requirements is going to leave a lot of gaps – and support will evaporate. Better to get going now and implement some visibility quickly and show what you can do – rather than what would be ideal to have.