I'm guessing somebody here has one of these things, specifically 64mbit SNES.

So, I tried to put some ROMs on it. But I only get about 40mbit before it says it's full.
I'm guessing the ROMs are rounded?
I tried my 3x3 Eyes WIP (8mbit, expanded to 12), Dream Maze (8mbit), Jelly Boy 2 (12mbit), and Lufia 1 (8mbit, the only nonhack, because I need to see if my friend can help me solder a new battery into my legit cart). Unless the cart automatically expands them all to 16mbit or something?

I'm guessing only one ROM can use the SRAM at a time?
(if I save in one game, then in another game, the first game's save data is destroyed?)

(don't know if anyone's tested the JB2 patch on real hardware before. Just curious if the broken credits music is an emu bug or the state of the proto ROM. I know it's not the patch's fault.)

Those games cannot run without the SuperFX/FX2 chips, respectively. That should answer your question.