Just my observation with my last twinscroll manifold and T3 turbo... They weight about the same. Schedule 80 stainless pipe isn't light by any means, and it looks like that manifold has some length to it. Some people like to debate the extra downpipe length, but we all know that it's made with thin-wall and mathematically insignificant.

If you do want to play the weight game, I'd be more concerned with putting a heavy turbo with a heavy manifold on the same side of the car as the driver. That's probably not helping the cross weights.