68k software will run much faster under System 6 or 7.0.1 or 7.1 than under A/UX 3, obviously, but if you have enough RAM, the software will probably run better under A/UX than it would under the pressure of the bloat in something like System 7.5.5. I tried period products from Claris on an SE/30 running A/UX 3, and, as I recall, they worked just fine.
More positively, I would say that in some ways, while an SE/30 is rather slow for A/UX, it is also a good machine for running it, because you aren't going to want to be using your collection of fancy-pants Nubus cards in the thing, and these almost never work under A/UX. To put this another way, A/UX limits a Mac II or a Quadra in a way that it can't limit the SE/30....
(By the way, SE/30 Ethernet drivers are built into A/UX, so you will be good on that score, and A/UX overcomes the 8MB RAM limitation of the SE/30's ROMs. At the same time, A/UX is not compatible with accelerator cards, so far as I know, so that would be a limitation.)