I have 84# at 29" limbs for buffalo on my Black Widow, but its a few years old and not as fast as the current models. I called and talked to all the manufacturers, before I went on my first buff hunt and most just said they had nothing that would work. Gold Tip didn't have the Big Game 100's on the market yet, but sent me four to test, I still us them with my 80# limbs for other animals, but I can't get the weight above 700 grains even in the 80# bow. I have killed lots of stuff with them, anything smaller and not as tough as a buffalo!
The carbon inside adds LOTS to the spine, normally I shoot the 2219's out of a 63# bow. I do wrap a spiral of cotton blend sewing thread around the GT, before I push it in, that makes it a snug fit and it dosn't "buzz" in flight, it also raises the spine a bit more, I'd guess about 5#.
To give you and idea of what is involved, I probably have $350 worth of failed buffalo arrow experiments sitting over in the corner of my office. Some of them in fact work great, but you just can't get them on a reliable basis and some of them are probably collectors items!
Cory Mattson, whos photo is at the top of this page, just copied my arrows exactly, he was using an 85# Brackenbury.
The problem with the majority of the big/dangerous game arrows is that they are designed for heavy weight compound bows, not trad bows, hence the reason the GT's "100+", supposedly they are for bows over a 100#'s.