a pilot should expect the same level of service irrespective of controller/FISO workload
Nonsense, it's covered in the ATSOCAS CD (also available online). Under "Service Principles - Reduced Traffic information" where it lists the situations when it may be required to reduce the level of traffic information passed. High Traffic Density, Controller Workload, Operating close to the limit of Surveillance System coverage and when using SSR only.
If one day you tell the pilot about traffic and the next day you don't, the pilot will not know which day he has to maintain a good look out
On WHICH DAY???
Every day, inside CAS, outside CAS under any sort of service, the onus is on the pilot to maintain a good lookout!