I've replaced my cast iron control arms recently with aluminium arms. Had to exchange the front Powerflex bushes and the small piece of the rear bush from the SuperPro anti-lift kit bushes. Also replaced the ball joints for the S3/Passat which correct a bit the roll centre.
The aluminium arms weight half of the stock version and my suspension with a BBK that has 13 kg brake discs became less crashy over bumps.
It's only worth it if you can source the parts for a cheap price. New items are a bit expensive, I think.