I have branches on my Iron Maiden. Does it make a difference compared to a standard cast iron manifold?..... I have no clue. But my branches are no where near 76mm, in fact I doubt if they are even close to 63mm. At a push they are probably 50mm, and then they all meet into one cluster with the flange to mount the turbo onto. Is my set of branches correctly made .... I have no clue. The 6 lengths all look identical.
Maybe on a 3L, 4 cylinder the exhaust ( not down pipes )can be a 76mm reduced to a 63mm. Personally a cannot see why though, other than creating sufficient back pressure to prevent the turbo over boosting/spinning. But on a 4.2L, 6 cylinder the motor doesn't rev enough to overspin/boost.....hahaha.... and currently my turbo has a 2" outlet on the outlet side of the turbo which then increases to a 3" pipe and stays that diameter all the way.
Petrol motors work VERY different and need a little back pressure to gain the efficiency needed, but a diesel motor needs to breathe, but definitely a good balance needs to be achieved!!!