I haven't used BF specifically, but I am told that BF and WG are extremely similar (like Richt said), and I have used WG quite a few times.
I don't find myself topping anything anymore. For a while, I was topping products to just see how they would come out, but like Accumulator said, once you top it, you are stuck with only being able to add more topper and no more sealant. I actually like the look of multiple coats of a sealant over a single sealant coat with a single wax coat. In your case, if BF in fact does look like WG, I'd say to add a couple more coats of BF before even thinking about topping it. You will be in for a treat after the second and third coats because it just keeps getting better.
Once you get to a point where you don't find layering more coats is looking any different and you just *have* to put something else on, then at that point, I'd top it to preserve the finish.
As far as what look you will end up with, I would think that you would add a little more depth and gloss but will lose some reflectivity and shininess, which always happens when topping a sealant with a carnauba. The BF sealant already looks a lot like a carnauba (hence it's darkening properties) but with a wetter finish, so it's going to be hard to beat.