Using indirect KPIs is always a dangerous distorter of behaviour. if you want the SLAs to ensure the appropriate resources are applied and to drive the size and location of teams required and the spare part/hot swap stock size and locations, then write the SLAs so they define the appropriate resources to be applied by priority of incident for that service and define the size and location of teams required and the spare part/hot swap stock size and locations by priority of service. Don't make the behavioural causal chain any longer than it need be - you'll get all sorts of unintended consequences.