Idly, someone from #68kMLA did this and they got
great MacBench results, so although there are some quality of life things that aren't so hot, if you need raw performance for, say, rendering something in a video/compositing/3d app, or something graphical that doesn't necessarily involve sound, or you have good control of the sound in some other way, as indicated above.
@jimjimx Did you overclock from 1.25 or 1.42? Those are 250 and 80MHz respectively. It's arguable whether or not it's meaningful. Especially when, as an OS 9 machine, any G3 is "fine".
Re 1GB of RAM: That's normal for OS 9. I don't think Apple ever imagined a use case for it, and to be honest, I think they're correct: there's no real use case for over around 256-512 or so megs of ram on Mac OS 9 where, from a technical perspective, you aren't meaningfully better off moving to Mac OS X.
Having that much won't really hurt anything, I've got a couple OS9 machines with a full gig and they run fine, even though I never need that much, by a long shot.
Overall, a Mac mini would be a good OS9 machine, but in general, if you don't need a powerhouse for period-correct RAW photo batch processing or video work, a Power Macintosh will run '90s software "fine", and most Power Macintoshes (save the first generation, if I remember correctly) aren't yet at the point of needing capacitor work.
That said: A mini is also smaller than most of the other machines, save PowerBooks, which can be troublesome for different reasons.