Not sure fast is an issue, where you hit is more important! Might consider single bevel broad heads from all I see! Lots of practice from all positions, angles, and be able to hit that "grapefruit"every time! Actually the vital area doesn't change, the smaller zone tests your skill better, IMHO! Try the Finnish Hunt test, 3 arrows in a 10 cm circle at 18 meters. Like Hegde said earlier, where the first arrow hits is most important! Pat B also had some good advice in another thread! However, I am an instructor, not an experienced bow hunter! Maybe, this year!
Hawkdancer