This is getting interesting, Jim. I focused on RealTracks because the regeneration of a song only stresses the CPU and RAM if the BIAB song has RealTracks. The slowest computer out there can play back audio files and MIDI files.
What you say is mostly true Matt but remember every midi track requires a synth for audio playback. While the actual midi data may not stress the system, a soft synth used for audio playback may. Also there are other types of files like KAR files that include lyric data which can add additional system stress.