Yes, it shows 20MB, but how much of that is free? If you have "dirty" ROMs, and MODE32 is not installed, the extra RAM will appear to have been used by the system.
IIfx ROM-SIMMs are 32-bit clean. The only issue you'll have is a bit longer of time to startup the SE/30. Sometimes it can vary depending on how much RAM you have installed. I've heard of cases that had wait times of near 5 minutes before booting.
So let me get this straight. If I have "Dirty ROMs" and I have 20MB installed. Will it show as having 20MB in the About Finder menu? (but in fact only have 8MB usable until I use Mode32) or should it show 8MB until Mode32 is installed?