How fast and light is an interesting question. We have a lot of customers that have been taking this to extremes farther than I ever thought of. Guys have shot big animals with what would be considered very light bullets with stellar results. We all learned to use heavy for caliber bullets for good reason. Go back 20 years ago and there were not a lot of bullets that would survive launches much over 3300 fps and if you could get that kind of vel and had a close range shot there was a good chance of bullet failure on impact with little penetration and a train wreck. The logical solution was heavier bullets to get the vel down and hopefully have enough bullet left after a high vel impact to penetrate deep enough to get to the vitals. Well, we don't have any trouble with either issue. We have not been able to impact a bullet fast enough to get a failure, nor have one not survive a launch.
We are long range guys so it must be said, there is always a break point down range when the slower, heavier, higher bc bullet will pass up the performance of the lighter, faster, lower bc bullet. This has to be looked at on an individual basis and what the goals are for a particular rifle/hunter. Often this break point is farther down range than the individual hunter is wanting to hunt.
So when using a bullet that has no high vel launch or impact issues it is a whole diff world of terminal performance. The Shock of high vel impacts can not be denied. High vel impacts with a Hammer Bullet that rapidly deforms and sheds the nose imparts a shock (not energy) that is simply stunning. Then we still have the same retained weight that will penetrate deep to reach the far side vitals so that you don't have that animal that collapses from the shock but then recovers to jump up and run away due to lack of vital damage to bleed the animal out.
@ButterBean and @farleg (not sure if he is on lrh) have more high vel impact data on animals than anyone else that I know. Some of their data could be considered controversial so if they are willing to share please don't blast them with any self righteous flap. The on animal data that they have gained is valuable and can't be duplicated any other way.