The refresh/update speed is decent if you're using black and white or minimal color depth. You can do high color depth, but the performance gets exponentially worse. (ex: 16-bit color is 16 times bigger than a black and white frame buffer)
The way the PowerView works is to copy the entire framebuffer over SCSI to the PowerView. The graphics chips 2D acceleration capabilities aren't used at all (as far as I can tell).
The SCSI video implementation is not complete and doesn't work very well. I have personally never tried it.
Different people have had different luck. As long as you use the right Mac driver, it can work fine.
How good is the refresh/update speed? I'm assuming that would vary based on resolution & color depth, but could it be used for something like Warcraft I? Or SimCity 2000?
ShufflePuck would be great on it (black & white). The resolutions are locked in at the early Apple monitor resolutions. IIRC, 600x400, 640x480, 800x600, maybe 1024x768? Newer versions of the MacOS driver might be able to do better, but right now the PiSCSI implementation will only work with version 1.0 of the driver.
The Pi has memory and may do thousands or millions of colors.
Pi memory definitely isn't a problem. Since the hard work is done by the Mac, its Mac memory that is the limitation.
As for the portion of the screen, that may be normal with HDMI monitors. I have the same issue when I use the lower resolutions at thousands of colors on my P476 on top of my HDMI monitor the screen doesn't fill up the whole screen and is slightly offset. When I use 1024x768, the screen fills the monitor but is offset and limited to 256 colors on the P476.
I didn't ever get around to screen positioning
Just need to figure out the Linux-Fu to change the framebuffer resolution on the fly.