I have shot several elk with the Sierra 180gr. bt, and they perform extremely well. My velocities generally were at 2960-3000. The only thing you will notice is more meat damage than using something like a Barnes, or other bullet which holds together with less fragmentation. They will generally produce a quicker kill than the bullets which tend to have exit penetration. Having said that, most of the time I have found the bullet just under the far side of the hide with the Sierra.
The elk shot with the Sierra 180gr. have all been their spitzer boat tail. I have not used the match king on elk. I have contacted the techs at Sierra and spoken to them, and they all say the expansion is unreliable. Although they may perform well, they may fail to open up or fragment. The SBT gameking has a high enough BC that except at extreme ranges you would not have any real advantage. I live in New Mexico, and both guided and hunted elk for a large portion of my life ( I am now 58). The vast majority of elk are shot under 200 yds, and quite often under 100 yds. There is no way I would take a chance on shooting an elk and risk a foul up by choosing a bullet not designed for the type of hunting I am doing. There are numerous bullets which will perform well. The Sierra SBT is acccurate, has a high BC, and performs well on game. As an aside, I went out with reloads and was sighting in a new scope I was placing on my 300 Win mag. I shot some 180 gr. SBT I had reloaded 4 years ago and with a cold barrel shot a 0.4 in. 3 shot group at 100 yds.
I have shot more than a dozen elk with the Sierra 180 Game King boat tail out of a 30-06. Both the spitzer and the hollow point work just fine. I am now using premium bonded bullets exclusively for the better BC and controlled expansion.