I don't really understand who would sign up for an exchange tour with a tour plus five year return of service.
I am guessing that these guys are experts and are/were probably qualified Nimrod guys. Would a three year tour really be enough to get you to agree to a 8 year (total) return of service at that stage in your career?
I doubt it. But a promotion or specialist pay scale or (dare I say it) FRI might.
Less stick, more carrot!