An 8x42 will provide roughly double the light transmission of a 8x30 binocular. On the other hand I would also consider a 10x42 binocular. Handholding a 10x binocular is a bit more difficult but you get better visibility for bird ID in low light with the extra magnification.
I always think back to the song with the line "how hopeless it all becomes when you seek all the answers in one". I have a 7x50 marine binocular that is almost like having night vision binos out on the water at night, a 10x25 pocket bino, and Swarovski 8x30 bino, Pentax 10x43 bino, and an inexpensive Nikon Monarch 7 8x25 bino that I leave in the one car and a Nikon Monarch 7 8x42 pair that I leave in my truck.
The pair I use the most are the Swarovski 10x25 CL Pocket Mountain as they fit in a jacket pocket or in their case attached to the side of a backpack and are the most convenient to have on hand. In low light my pick is the 10x43 that works the best for me.
Your point of double the light transmission is highly exaggerated. 8-P
Where did you get that ?
All binocular models are different in many ways, I will leave it at that.