If you are on NE4 firmware, with a touchwiz rom, I know flashing the 4.3 modem will not brick, but it will not work (no signal). Its just a guess until confirmed, but I'd bet the same will happen with other modems and roms (dont know for sure on cm/aosp though) This basically means you are stuck using the NE4 modem im afraid....
---------- Post added at 10:58 PM ---------- Previous post was at 10:54 PM ----------
Once I find someone to test it, and provided it works as it should, ill be posting the odin flashable full NE4 firmware. You might want to try that if you see me post it. Otherwise, im not really sure what else you could try...except maybe flash a different rom (4.3 roms would be goo