Yeah, I'm aware '030s will boot 8.1, unofficially, good that you've made the necessary adjustment in your images. I'd be curious to try it out on such a slow machine. In my experience, running too high an OS version on a machine can impact performance. (1)
On '040s: 8.1 itself runs fine on my 840av with 24 megs of RAM, I believe you can get a usable system folder with OT/AppleShare and some other niceties in about 10, maybe 12 megabytes, but you'd want to be running early-mid '90s software (not mid-late '90s software) in order to economize your use of the rest of the RAM, so in reality 24 megs is close to the bottom end of where I'd want to be on a system running a bunch of software from 1998.
Overall, my preference tends to be for 7.6.1 on most of my beige Mac hardware, but I may ultimately settle for 8.1, particularly on my 6100 and 6200, to decrease the amount of 68k code I'm running on those machines. However, in addition to that, there are a few quality of life improvements in 8.1 over 7.6.1 for networking specifically (multi-threaded finder in particular is a huge benefit for both local file operations and network-based ones so I can see why you'd want 8.1 on machines where you're handling a lot of files.
Regarding space "efficiency"
1) You need a Quadra to have over-4GB disks at all
2) In my experience with 7.6.1 on my PowerBook 1400 using a 30-gig disk, file sizes don't get too out-of-hand, although that system is 3-way partitioned, so my largest volume is, if I remember correctly, 15 gigs.
3) You can access an HFS+ data partition, but not boot, using 8.1 on a Quadra. You need PowerPC to be able to boot HFS+.
4) If your files are "big" anyway (literally anything above plain text files, really) then it won't matter. Even 640x480 PICT and JPG files are going to be big enough to make most or all of the difference.
I'm not 100% clear on whether or not the ROMinator has any fix for this, but I imagine it does not, because if it did, people would be shouting it from the rooftops.
My recommendation would be to make a handful of 2-4 gig partitions on your SCSI2SD (you can even split one into multiple devices with their own SCSI IDs) and then use localtalk or ethernet to store more files on, for example, your 9500, to which you can add a SATA card and use 2TB disks.
I have a couple machines set up using LocalTalk (my PowerBook 1400 in particular) and although it's particularly annoying because I have to copy files in two phases (I should really just set up a software LT bridge, or get one of those phonenet terminators) it's Fine(TM) even for a fairly fast machine.
(1) Though, my experience with this right now is with OS 9.1 on a Power Macintosh 6100.
