After reading this thread and checking out the new arrow offerings from Terra Firma again, I'm into the idea of dropping back from 530gn to about 500gn again. I know the difference in the long run isn't really that big, but there is a chance I may be making slightly longer shots at fallow bucks and red stags over our winter months, so an extra bit of speed for more forgiveness in my trajectory might make a difference.
If the chance at water buffalo comes up for me I'll just go to a slightly heavier arrow again, but I know of many people who've shot them with 70# and 500gn and they've been just fine. Me shooting 78# should be more than enough.
I've never really felt the need to have an arrow of less than 450gn for the sake of more speed and a flatter trajectory, but with my 28in draw, if I were shooting 70# on a slower bow, I can see how others may want to go that way.