The issue with performance pay for heads of quasi-judicial tribunals is the legal difficulty of having the government assess performance of someone who has an investigatory or decision-making capacity vis-à-vis government decisions. There is case law to that effect saying that it is seen as going against the required independence of the agency for the government to have flexibility in what amount should be attributed as performance pay. This regime of having payments made in recognition of management responsibilities for heads of quasi-judicial tribunals was created in 1990 in reaction to the cases that had occurred in some provinces. That's the regime we have in place.
On March 26th, 2007. See this statement in context.