All FT cards and all Sonnet cards sourced from either ACard or FT do boot MacOS 9.x.
Under two conditions:
1) The card has to have PCI or PCI-X interface. Obviously, the cards with PCIe do not boot MacOS 9.x.
If someone can hack the 2005 G5 to boot MacOS 9.x that would not help either because these PCIe cards do not have Open Firmware bootstrap code.
2) The drive in question has to have a generic MacOS 9.x SCSI mass storage driver partition. Without that partition the drive will be seen by Disk Utility only, but won't show up. That partition can be installed by the usual Disk Utility and it's siblings (FWB, ATTO, LaCie, etc). If not, there are some system extensions which are a generic SCSI mass storage driver in disguise - these will usually work, too. YMMV.
The only notable exception from the rule above are the PCI-X cards from Sonnet based on Marvell 88SX.... chipset.
These aren't bootable on either macOS X or MacOS 9.x
And some interesting addition: the originator of probably fastest first generation PCI-X SATA was Vitesse, not Intel. Even more - there was as I remember an other company who engineered these chips, than Vitesse acquired it, than Intel did partner with Vitesse. They made SATA based on some existing technology which had nothing to do with SATA and delayed. At the end Sonnet was p***ed off and turned to Marvell. But they wanted the Vitesse originally. That was sad.
Ultimately the Marvell turned to be the best choice before AHCI swallowed everything.
AHCI is really a good standard - what made G5 and all pre-Intel Macs difficult to expand was the fact that the best that-time AHCI controller (Marvell 88SE923x) had only two PCIe lanes and the architecture of 2005 G5 made it impossible to use it there.
I am guilty that I did not try that Marvell controller in an earlier G5 or G4. But since it is a PCIe controller, I would need a "bridge". The cost of proper 64-bit wide bridging would be prohibitive.
This is why there is no AHCI controller on G4.
For G5 there is a single choice, based on Marvell 9128. Unfortunately the performance of 9128 is very disappointing.
For good or bad, I did not try others like ASMedia because only Marvell AHCI controller had FIS-based switching for Port Multipliers. ASMedia stuck to command-based switching. But... in the fact, the Marvell 9128 is barely better, I can't address within the same PMP more, than two drives the same time. Which makes the FIS-based switching on 9128 rather a theoretical benefit.
I wish there would be a 4-channel PCIe SATA AHCI chip, but there isn't. The on-board SATA controller of 2005 G5-s is very-very dated.
