With my testing value 256 made 1080p playback more smooth, with 128 it was stuttering too much, 512 caused small lags with other usage with desktop.
What values you are using for gaming and 1080p videos? I heard some retropie users are using value 320, but I think I need study more this.
Geez just release RPi 4 already with 2gb ram, so no need to think things like this, insta 1024 gpu mem.