It's not going to improve handling due to the weight placement. Cast trans is heavier, but you would see more improvement by just placing the battery in your trunk. Any weight shaved from the front of the car is like adding that weight in the back for transfer. If you pull a 40lb battery and place it in the trunk, it's like adding 80lbs to the rear for traction.
The only place a lot of weight in general hurts you is it's ballast creating inertia and fighting it while stopping and then accelerating, like in traffic, but a transmission isn't where you want to look.
I went out of my way to find an iron case for my gears, because gear drives like spreading, which is exactly what happens. Main and countershaft gear sets want to force themselves apart under tension and it's a lot more likely for an aluminum case with a floating countershaft to allow it to move, than an iron case with a fixed countershaft.
This is also why VW and Porsche H air cooled engines have problems with blocks needing align bored for the crank. All of them run timing gears on split blocks that are aluminum.
Aluminum tailshafts are fine, but make sure that the yoke bushing is good. The aluminum cases lost oil easier from poor sealing and inherently, lost oil at the bushing from starvation and they spin bushings. With an iron case, it should be ok. Aluminum bells are fine on a street car that isn't going to see high RPM and heavy clutch dropping.
