When the SE/30 came out it could only address up to 8mb of RAM anyways due to the 24-bit ROM. I think you could either swap the ROM from IIfx or IIsi (which came out the next year), or use the MODE32 INIT[0] which I believe required System 7 and came out in 1991, two years after the release of the SE/30. I guess 128mb of RAM would have been a bit cheaper by then... hopefully haha

[0] https://en.wikipedia.org/wiki/MODE32