I took out about 100lbs. It made things a bit lighter, but the rear strut brace made the big handling difference.
These cars suffer from a lot of flex. It's like driving an unpredictable cylinder. That brace made it much easier to flick around. I can hold the back end out all the way around a turn without fear of it completely spinning around.
I relocated the battery to the trunk, built that box, and my full size spare, so I added about 50lbs back, but also less weight in the engine bay.