I've got the Mathew'sLegacy. Shoots great. Had a PSE XLR 900 @ 83#'s. It'll send arrows thru a hay bale no problem. Legacy shoots just as fast, but w/o the KE. Still havn't had a chance to let the air outta anything. Maybe next season. Or pigs in the spring.
The goal of the arrow is to be as accurate out of the bow as possible. Once that is attained, then you can begin to fiddle w/ speed vs. KE. More KE = deeper penetration in game. Speed = less chance for the animal to jump the string (duck an arrow). If your going after pigs or elk or bear, you'll want more KE to get to the vitals or a complete pass thru. Speed also helps to compensate for errors in yardage estimation.
Some of the most fun you'll have w/ a bow is going after Javalina in the desert. Season starts Jan 1 for archery in AZ. Its a great time to be in the desert...a really beautiful place. Plus Java's eyes are so bad you can sneak them to 5 yds. They charge too...my buddy thumped one out of a herd...they all split up, then came rushing back at him! It was funny to watch till he was outta arrows and using his bow for a shin guard