How bright would Sagittarius A* be from Earth if it became an Active Galactic Nucleus? Essentially I'd like to know how bright our central black hole could appear from earth in relation to full moon brightness. Assuming maximum energy output (like when it's gobbling up an entire globular cluster). I've read that the light from an AGN could actually support photosynthesis on planets within 1100 light years, so I'm curious if it would have any affect on Earth life 25000 light years away.

https://aasnova.org/2019/05/29/do-active-galactic-nuclei-help-or-hurt-life/
If Sagittarius A* were to ignite and feed on a massive accretion disk, blasting the massive jets we call quasars, it wouldnt be noticed by anyone here on Earth unless they were looking for it.  The relativistic jets accelerate out of the poles of the Supermassive Black Hole (SMBH) shooting out of the plane of the Milky Way, not through the disk structure which would cause massive damage to the galaxy. And with the naked eye, relativistic jets wouldnt be visible to us through all the gas and dust of the Milky Way, combined with the 26000 LY away from the galactic center we are, we really wouldnt even know the quasar came to life, and the Moon would still be the brightest heavenly body at night.  

For more information, check out the Wikipedia article about quasars.