With proper placement the 85 gr TSX out of a .243 WILL take an animal the size of a cow elk, BUT...I would not consider a midweight bullet in a .243 to be anywhere close to ideal. I would look at the .270 150 gr or better the 7mm 165-175 gr to be a far better choice.
Will it kill? Sure. Is it humane? I doubt it.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball