In terms of managing an OS/2 network of storage devices, what would be the most effective filesystem for speed and efficiency?
* If I read this question correct, you have OS/2 systems as user interface (aka clients, personal conputers..), all networked together and you look for a solution to provide a common network storage for these systems.
The easy way is an OS/2 file and printer share-network, like the same in windows environments: you can use bulit-in tools from OS/2 to access them. If you want the hard way, you can use FTP (not very user friendly to integrate in WPS), the most efficiency way is to use NFS, but have to buy something like the Netdrive-software. Thats all on the client-side. You talk either SMB, FTP, NFS (or some other Prot.) over network to a networked storage system (NAS).
(I know: nothing new here)
The storage systems today all talk these protocolls by default (at least: this can be choosen). What kind of a) file system, b) OS, c) hard disk controller these NAS using, is unimportant for the speed and efficiency from the clients point of view.
The clients are measuring speed by network link speed, throughputspeed of the used hard disks in the NAS and the possibility of some bottlenecks in the controlling part: (e.g. the NAS is fitted only with a cheap ARM, a simple SATA controller and to few RAM, but should encrypt the entire communication as SFTP, and running the stack of 6(!) disks as a softwareRaid in Raid mode 6.

Okay this might be the worst case

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The clients don't know (and don't have to), what real file system, OS or hardware sits on "the other end of the network cable", they got provided with directories and files. So you can buy a commercial NAS system (from a simple Ethernet-aware hard disk standing next to your pen holder cup up to Systems like NetApps FAS series, worth more than a middle class car.
They all work the same ( in general). The hard disks canbe simple kinds of SATA magnetics, Hardware-Raid-connected SAS-SolidStateDisk or Bus-connected FusionIO-SSD-Cards, is just a matter of speed and money. You can even build a "NAS" on your own, based on a PC architecture. You can use Windows, Linux FreeBSD or even OS/2 as OS for such an system (okay OS/2 is handycapped in many ways in this use case).
You can manage your entire storage from monitoring single hard disks up to glueing together network drive space in several modes (from "just a bunch of disks" to a "Raid 1 mirror of several raid-6 glued harddisk groups", resulting in something like "if 3 disks fail simultaneously out of 8, you are still on 'Go!'". Managing this will usually done by web browser.
You are mentioning several disk sizes, that indicates older or recend single hard disk drives. You can take them as your NAS drives and can integrate them one by one (I further assume they are formatted with HPFS or JFS. Both cannot used in most NAS systems, so you have to reformat them in the NAS box).
But I personally suggest to substitute every harddisk you have at least all 5 years, maybe sooner, copying the data. If they already run in a Raid 4,5 or6 environment, you can wait until one drive failed to substitute them, but you should have the spare drive handy.
* If you want to set up your storage within your computer an not network attached, your are faced in most cases with the common known SATA-drives. You can take Sata-connected SSDs to speed up your storage part of your PC, but if you are take OS/2 as your OS, you are limited to certain file systems. All of them are not fault tolerant of disk drive failure, even our LVM doesnt help you in this case. We dont have Software Raid technologies at hand like Linux or windows, and the use of Hardware Raid controllers are limited to very few old SCSI, if any.
You can do a cron-based copy from one drive to another, but thats just a cheap solution thats even have to handle files like os2.ini exceptionally.
You are mentioning ZFS from Oracle. I assume you work with this an another (non-OS/2 -) network. In the OS/2-world, there is nothing like this. LVM was designed to do such tasks but our LVM in OS/2 is far behind the possibilities of the AIX-LVM, and even further away from ZFS.
* But in a OS/2-System, the speed of a build-in SSD cannot reached with any of the NAS technologies out there. Simply because we are stuck by 1Gb-network cards that we cannot bridge. There are 10Gb cards out there but without OS/2-drivers (The world-demand of this can be pobably counted with the fingers of one hand). So my suggestion: set up your clients on simple SSDs, make a backup every time you start to a SOHO NAS. Buy 'Netdrive' with 'NFS-Plugin' and put all shareble files on the NAS.
* And most important: Backup up your old SATA-drives before its to late. Even if you use a grml-LiveCD with the ddrescue-command without ever mounting your file systems in Linux.
just my 2 cents