This summer, I bought a 1K to defend my birdfeeders from marauding squirrels. Being an airgun neophyte, I thought that no airgun could take out a squirrel, which I considered to be a de-facto armored creature. One day, though, a bushy-tail was foraging about in the back-yard fur exclusion zone, and I popped 'im in the butt ... just to spank him. (About 10yd range, Beeman Silver Bear round.) He successfully fled up a nearby tall tree.
About 15 minutes later, my wife was out in the garden, and said squirrel's carcass plummeted from the tree and nearly landed on her head. Close inspection of the body revealed that the pellet had entered high (i.e. dorsally, near the spine) in the right flank. The round probably nicked the big IVC vein, causing the varmint to exsanguinate.
Five bird feeder bandits later, I can attest that the 1000X will eliminate squirrels at short range. I aim for chest now; with a hit in the heart/lungs, they'll depart life in about 10 sec. I'm not yet a good enough shot to hit them in the head, but if I could, there's no doubt in my mind that they'd expire instantly. (I am striving to improve my aim with regular target practice.)
The owner's manual lists muzzle energy at 17 ft-lbs. If y'all are getting independent velocity measurements at 970 fps, then 17 is not much of an exaggeration, and the weapon is a pretty powerful one ... at much less bucks than top-end rifles that sport a similar punch.