I agree that there is a lack of long term studies into fatigue.
Based on personal/business experience tho' I have to say that very long fights/RTW flights on a regular basis seem to be affect people over the medium term. Just about anyone can do it for a few months - the insidious effect occur over the 12 -36 month period where some people start to suffer significant behavioral changes.
Companies I've worked for have often observed that most people can work every day for 3 months without any harm, 3-6 months they are still doing OK but there is a gradual change in some behaviour. 6 months plus and you are getting onto thin ice. Same with Ultra LH in my opinion.
The question I have is will QF pilots absolutely refuse to conduct Ultra LH without all the necessary break-periods etc being agreed or will they agree to do it if the Company offer $$$$$$$$?