Isn't that a quite a lot faster than a FW400 bridged disk?
Numerically: yes.
In practice: Probably not meaningfully so.
For consideration: it depends on use cases and on the type of drive you have.
Hard disks never had the giant performance uplift SSDs had.. For example:
https://www.storagereview.com/seagate_ironwolf_pro_nas_hdd_12tb_review the rated maximum sustained transfer there is 215 MB/sec for one of the biggest, newest disks you can get. It was SSDs that eventually reached the upper limit of 6-gigabit SATA performance. A performance-oriented hard disk like this one
https://www.storagereview.com/seagate_barracuda_pro_12tb_review can sustain 250MB/sec.
I looked at an example 1.5TB disk from when those were shipping and a Seagate 7200RPM disk of that size I found on Amazon (used, probably, since that size was common around ten years ago) was rated for about 125MB/sec, so a bit between the two interfaces.
SATA that works in old Macs is rated for about 150MB/sec. Firewire 400 is up to around 50.
At some point I'll look, I have an FW400 enclosure I mean to put some 2TB disks into, but I'm not expecting it to create a meaningful difference.
You'll almost certainly notice more if you're doing something that causes a lot of random, high-speed reads and writes, though again as I mentioned about SSD in general, if you put an SSD on firewire and a big spinning disk on SATA the SSD would almost certainly still
feel faster.
I'm largely unfamiliar with what faster SCSI devices can
actually deliver.
(If I'm reading
this right, a high end disk from 2004, a tad under 75 MB/sec in the best-cased scenarios. That number probably improved with a good RAID controller and using a few different disks in a stripe.)
TL;DR - I struggle to believe having storage connected via Firewire instead of a PCI SATA card would matter for almost any OS 9 use case.
Illustrator is CPU limited and the files take up no space as compared to your graphics workflow and storage requirements which are more disk limited in time and space than CPU limited, no?
It depends. Right now, is there a delay when opening files? Do you ever spend time rifling through files trying to find a particular version? Those tasks would be faster on an SSD, because of the seek times.
For photos - largely, disk access and seeks have been the death of almost every system I've had. It's why my G3, G4, and Core1Duo[Rosetta] Macs all appeared to be working at about the same rate, with Photoshop CS2.
For video - with DV, keeping a disk clean so large files can be read sequentially at speed is what's important. If I were configuring a DV/HDV editing computer on an old Mac, I'd probably do dual disks and keep a cleaned-up capture/render disk. The DV bitrate is really low, around 13 megabits, so it doesn't really matter what disk you use as long as it's in working order and
Today SSDs became important for modern video largely because files are so big and data rates are so high individual spinning hard disks can't keep up with editing workflows.
There are FW/USB adapters.
That said, given that you've already got a working setup, I'd say keep working with what you've got. A couple people I've discussed this with have tagged "building an OS 9 machine cheaply and easily for someone who doesn't already have one" as one of the more exciting things about OS 9 on the Mac mini, but 9 works well on so much, you'd be better served with that as a solution if you were reasonably familiar with OS 9 but needed some kind of machine for a very small space.