Anyone tried an MC68030RC50 in their SE/30? Did the computer explode and you lost your eyebrows in the flames and now you have to draw your brows on? Orrr... any other bad stuff happen?

I know I'll need to get a new crystal oscillator. And according to cpu world the full 030 still relies on an external FPU, which I assume will need to be at the same clock speed.

Nothing bad happens. The CPU won't even break a sweat since you're running it a half its rated speed.
That is if you don't swap the crystal.
Understand that there's no such thing as a "CPU clock" on these old things. If you swap to a 30MHz crystal you overclock the entire logic board including all its components! Even stuff like the serial port and the PDS slot.
The SE/30 does not overclock at all well, and lots of issues are known to occur if you do.