Well at least in terms of the 971s, it is. 6G is flash/mirrored gray and 8G is gradient gray. But exterior flash, in my experience, doesn't assist with harsh sunglight at all. It doesn't darken the lens looking from the inside out, which is what it sounds like you're after. By the same token, going by past experience with other Bulgari lenses, their flash gray lens is not necessarily lighter than the gradient gray, either. They are about the same in terms of "darkness", it's just that one has a bit of a reflective coating.
I can tell you this: I tried on the 971 6G ones they have. They seemed pretty dark to me, and I think they could easily be worn in harsh sunlight. I know the Florida/Caribbean sun pretty good
What I don't get is if you consider the 971 6Gs out of the question, why you would even consider the 732 6Gs. They're essentially the same lens, just different arm colors.
You're wasting time trying to achieve the ultra dark look you think you see in the pics of Bono, and you're not going to get that out of either of those pairs imo, because you'll never be wearing them in that type of lighting. (Having said that I'm not aware of any pics of him wearing them in bright sunglight, either, so I could be wrong..)
ETA: considered the 966 8Gs? They look pretty dark, also.