The speed (only 3 Gbit) is a compromise, but on the other hand these cards can handle >2 TB drives.
both is not required for OS9. just saying.
Yes, but it is possible to get into a trouble. So special care has to be taken because realistically even hard-core users of MacOS 9 may want to boot into macOS-X. Which may reside in the same partition. On a disk which macOS-X does "see" as 3TB - but Open Firmware does see only the first 2 TB.
Here I am not 100% sure because the last time I touched Open Firmware was over 10 years ago - but as I remember, I always got 32-bit LBA addresses in the I/O requests. That means: the highest LBA address is 0xFFFFFFFF. And since OF would deal with 512-byte sectors, the limit is 512 * 0x100000000 bytes. That gives us precisely 2,199,023,255,552 bytes or 2TB.
Apple went around this problem by introducing within Open Firmware their own 48-bit calls inside of their own flavor of SATA (I am talking about MDD and Cheese Grater machines). For a third-party it is not safe to use them and these calls could vary from model to model. What remains for us is the generic drive package with as I recall max. 32-bit addresses.
Regarding the 3Gbit speed limit - that means you can't transfer over ca. 250 - 300 MB / Sec. The 64-bit 66MHZ PCI is capable of about double.
A single SSD means that you saturated the SAS phy of the controller without saturating the controller bandwidth.
So people would have to live with that. On the other hand the hardware itself becoming very cheap. The main business would be probably popularize that solution. I remember the time when a decent 4-channel 150 MByte/Sec SATA-I controller would cost close to $200. Today you can get a decent 12 Gbit (say, 1 Gigabyte / Sec) SAS controller for less than that. The same time a decent SATA-III controller is at least $50. Except that the SATA-III controller is only PCIe 2x and port multipliers are far more expensive than second-hand expanders.
I got recently a 16-port IBM expander card with fail-over for under $15. Try that with a port multiplier...
This one - works perfectly:
https://www.ebay.de/itm/IBM-46m0997-ServeRAID-Expansion-Adapter-16-port-SAS-Expander/192326557905Don't be afraid of the PCIe-looking bus of the expander. It is just for the power. Right now I am using a very simple adapter to power it:
https://www.ebay.de/itm/USB3-0-Pcie-PCI-E-Express-1x-bis-16x-Extender-Riser-Kartenadapter-Netzkabel-60cm/132428653550The SAS expander goes into 16x PCIe slot which is just hanging in the air. The power cable is attached to the motherboard-based power connector of 2009 Mac Pro where normally the high-end graphics is powered from, there are two of these even in G5.
The graphics card is the lowest of the low GT-120, in this project I don't care less about the graphics.
Some collection of SAS cards, the said IBM expander included on this picture. One extra with hopefully Open Firmware coming from a French seller, an other one with OF from Germany and one with PCI-X instead of PCIe coming from Russia. All slightly different flavors, it's all-you-can-eat SAS time for me.