If a reasonably price fare is available to purchase a business class seat then I would book that immediately and upgrade to first. I have just checked EF and there is Z class availability on all legs! As a bonus it will cost you less in miles as you will be upgrading all the way through and not sector by sector. Also, there will be no extra tax to pay either as already booked into business. Oh and all flights are A380's!!!!!

Sounds like a win win situation to me.