If it is 8.5 billion light years away and was traveling at the speed of light (100% instead of 99.99%) wouldn't it take 8.5 billion light years? Wonder how you got to the 850K years figure.
Yes, if you are assuming the jet was emitted right now. But the jet was emitted roughly 8.5 billion years ago. The light in front of the jet has already reached us. If the jet were also travelling at the speed of light we'd be dead right now. But luckily it's travelling slower than light so that's why we have 850k more years before the jet reaches us.
Does that mean, once the jet reaches us 850K years from now, we can say that will be a mass extinction event, or even the end of life on Earth? Compared to a billion years from now when the Sun's luminosity increases.
Matter decay will have significant impact on the mass of matter ejected by the jet, as well, particularly over billions of years. As it decays into a lower energy state, mass will be turned into photons, and less and less of it will be left to impact.
Plus, we won't be in the same spot in 850k years anyway. The solar system is moving around the galaxy, and the galaxy is moving around the universe, and space in the universe is expanding.