Why buy a GH3 over FF

The translucent mirror inside all of the new Sony Alpha cameras is going to give them an inherent handicap in low light, high ISO situations. Sony could update the firmware to do more aggressive noise reduction, but that is all. However, since you are using Lightroom, you can already easily do that yourself in post processing.

If you want better low light performance, get the less expensive Nikon D600, which also works great with Nikon's awesome creative lighting flash system. With either the Sony or Nikon, the Tamron SP 24-70mm f/2.8 DI VC is a great alternative for a standard zoom:

http://photographylife.com/reviews/tamron-24-70mm-f2-8