Hello Graham,
I'm glad that you found my information helpful. I don't have a Canon 6D myself, but I have researched the 6D and I understand that it is a great camera for astrophotography right out of the box. They perform great in lowlight conditions without generating noise and it has a full-frame sensor. It's also not an inexpensive camera and once it's modified, there's no going back. Do you plan on using the 6D only for astrophotography? I would try using the camera first in its stock condition, but using a clip-in light pollution filter. The filter will likely cost as much as the mod, but at least you have options. Also, I have been using my modded T3i with a light pollution filter installed.
Hope that helps,
Gord