Yeah originals are cast and allot of tubular are stainless steel, most people go tubular when swapping turbo's as they generally need a manifold to go with it. In my experiance stainless steel tubular manifolds are very hit and miss, I don't think allot of people on this forum run aftermarket manifolds but when I had a Starlet GT Turbo Toyosports manifolds were crap and I had one crack and another completely split at the joint and pump a load of black exhaust smoke all over my engine bay. Performance benefits are negligible really, pretty sure they're not restrictive on Audi's but I've known other car manufacturers to restrict branches to cut down power and stuff. So yeah personally I'd only swap one if I was going for a turbo that wasn't a direct fit and even then I might be tempted to get an adapter if possible. I'm sure some people will disagree with me and say normal manifolds hinder performance though.
