ORIGINAL: TFOX
The answer Greg is NONE,but I am also thinking that a fixed blade broadhead becomes less forgiving in flight at greater speeds.A head that isn't perfectly aligned will be magnified at greater speeds.
TFOX, I've been doing this for a pretty long time. I have always used fixed blade broad heads and have found that they group really well as long as I keep the speed around 275 fps. I took my old speed bow (321 fps) and added a heavier hunting arrow and got the speed down to 275 fps @ 64 lbs. XX78 2413 w/85 gr. BH. I believe the arrow weighs 445 gr, finished. They group beautifully.
I've been using Wasp SST hammers for the last 12 years because they are a tough, quality head and they spin test amazingly well. I only have to mess with about 1 out of 24.
Also, by limiting my speed to 275 fps, the 4 pins on my fixed pin sight (which I prefer) are not tightly packed together. The spacing is just right for seeing the target.