To be honest, I personally don't sweat much about it.
I know that, given the decent SSD, I can get around 60MB/s on machines with ATA-66 bus and around 90MB/s on machines with ATA-100 bus. Plus or minus. I'm fine with that.
Factors that affect drive speeds / speed tests in OS9 are:
1. Drives themselves, obviously.
2. Whether the drives are empty or have some files on them.
3. Drivers, i.e. formatting tools used.
4. SATA to ATA controllers (Marvell, JMicron or some other)
5. Controller implementation by manufacturer.
6. Interface type - SATA, mSATA or M.2 SATA (in case of JM20330 chip based adapters).
7. ATA controller on Mac motherboard.
8. Bus speed.
9. Processor speed. To some extent.
10. Drive size. Not always, but some earlier models had obvious write speed relation to their size. If it's over 100MB/s, it's not important for OS9 machines anymore.
SIDE NOTE.
My observation so far is that modern SSD drives with huge caches and Turbo Write functions are not much faster (if at all) than drives without them on native ATA buses in OS9.