Don't have time to investigate quite yet, and it's only recently Stewart posted the DMA-related commits. GCW Zero firmware has a proper separation of userspace from hardware, so even if they were beneficial on GCW Zero, it would require driver support to be added. Me, personally, I am too busy at the moment with other aspects of PS1 and drivers to focus on this. However, I am finishing some work on GCW Zero kernel framebuffer driver that adds BGR555 support, which is the native display mode of the PS1. So there will be some speedup soley from that, hopefully, in the next firmware and pcsx4all release. I've also added bicubic and nearest-neighbor IPU upscaling/downscaling to the kernel framebuffer driver.