Looking closely at
http://elitistjerks.com/f79/t130513-...s/#Set_Bonuses (link to the Set bonus part) might have gotten you the answer on your own =) anyway:
I'm guessing, for math's sake, you would exchange chest and head for the tier-parts, assuming a loss of maybe 300 agility and 400 secondary rating. Which makes up for about 2% of your total agility and secondary rating (comparing to my own gear at around iLvL 492), thus assuming a loss of 2% dps. Following Purge's results, which indicates a win of 2-3.5% DPS gaining the bonus, you can only win. (I also read somewhere, the bonus might give something about 4% or even more, but I didn't confirm that yet or see anyone do that)
Thats for the long and 'maybe incorrect', but lazy answer.
For the short and correct answer:
Sim it! (I love to say that)
Greets, Pyr.
/edit I should add that to the guide, as this is a fairly frequent question everywhere.