Derates, are more cost effective and also offer greater savings, these are then passed to the opperator by the eng lease co, generally in a lower cost per hour, u can work with them on this. Reduced thrust (assumed temp, flex) also saves money, but you dont tend to see the savings untill major inspections.
To give you some idea ive got a letter from snecma. ''from experience a 9% de-rate on t/o thrust would cirtainly lead to a minimum of 5% reduction in maintenance coats and up to 10%, depending on flt profiles''
Rgds