I have shot my share of water bottles. Its fun, but they don't provide a consistent medium for comparing various bullets for penetration. Penetration boxes are more difficult to build, and a pain to maintain, but they do give you more info. A friend of mine just uses sand bags, as he has tested various slugs in all kind of mediums, and knows what a slug fired into sand bags will do by comparison to using other penetration mediums, including pine boards. He has boxes of spent bullets and slugs he has recovered and labeled so there are very few bullets you can come up with that he can give a pretty good estimation on what it will do at various velocities in flesh. I have taken more than one slug to him so we could compare it to some of his own.
You didn't say how high your hit was on that Elk that walked another 200 yds. With any animal, and any gun, its going to be bullet placement, not velocity, or bullet weight, that is going to put it down. And then, different animals are going to react differently when shot.
It may depend on as little a thing as whether they just took a breath before they were hit through the lungs. If they have just oxygenated their blood they can go on walking even as their lungs collapse. A huge bullet, at shoulder- breaking speed, hitting the right place is just not going to guarantee that the animal will drop in its tracks. Only a central nervous system hit( brain, or spinal cord) will do that, and hunters who make those shot will generally admit they were not aiming to hit the spine or brain when they squeezed off the shot.
My recommendation is to go for accuracy, and not velocity( recoil) or large weight. Your 300 grain pistol bullet proved that it is of sufficient weight to complete penetrate the elk. If you can pick your shot so that it breaks a fore leg going into the lung and heart, or after it penetrates the lungs and hear, the Elk is less likely to be able to go anywhere.
As for tracking elk in timber, these are huge animals. Wounded, they rarely stay on a game trail, so you don't have to deal with other elk tracks that might confuse someone. Elk will break off branches in the pines leaving visible signs of the fresh breaks to see. There will be blood spurting from both sides, leaving blood on anything from the ground up to the height of the wound. Those tracks will be at least 4 inches long, and often up to 6 inches on a really large bull. Weight is going to be 400-600 lbs. and sometimes even more with the old bulls. That much weight is going to crush and disturb, and move a lot of pine needles.
Spend some time in pre-season scouting trips just reading elk tracks, and following one. The more you do this, the better you will get. Use the blood as confirmation that you are following the right Elk, but follow his tracks. A wounded Elk is also likely to show his injury in his tracks, by dragging a foot, or by his stagger as he slowly loses blood pressure. Its not hard to distinguish the tracks of a wounded animal from those of animals that are not hurt. The other animals will flee the scene, taking running strides. The wounded animal is going to conserve energy as he tries to find someplace cool to reduce the fever he is sensing because of the rush of adrenalin in his blood as it tries to stop the bleeding, while keeping the blood pressure high enough to send oxygen to his brain.
Good Hunting