As Cameron mentioned, the G1000 will be looked at in a future update, but for now, unfortunately, there is no way to adjust the brightness as it uses the default G1000 as a foundation for what we added to it. And, as of the TBM's release date (Last October), there is no brightness adjustment for it.
Just a sidenote, please don't take our comments as being "How dare you criticize us!". When I read that you were getting 55fps, and you wanted more, I have to admit, I had a headshaking moment, also. We wanted to really push the limits of detail and systems, while still getting acceptable performance on the average PC. I actually managed to cut half a million polys from the mesh during the first few weeks of release. Our benchmark was Saso's PC. I think he's running some ancient i5 CPU with a GTX 950 on a Linux OS, when most other users have something around the 1080ti level GPU and an i7 CPU. But he runs it with no scenery and very little to no plugins. With that, he can get 25+ fps on the ground.
In any case, happy to see your tweaks are giving you what you want. Enjoy.