Certainly it shows that the korean boot.bin is compatible enough with the US firmware. First I should explain a little about how the boot components of the firmware work: You will see from RGN_Tool's GUI that the boot.bin is flashed to a region "0C" and that's the decimal number "12" in hex, ie 0x0E. Region 12 seems to be a virtual region and it in turn writes data to 2 other, regions 5 and 43 from memory, which actually boot the device. So now we need to try to overwrite rgn12 again with the US boot.bin so in turn it can write the correct data to the other two physical regions. You'd maybe think, "Well the Korean data overwrote them ok so why wouldn't the US data now write back just the same?". Unfortunately it's not always that simple and many things can go wrong because you're trying to write to regions which are being used in the process. It might work and it might not. It might even kill the device completely. In short, you now have two choices: (i) Try again to flash full US/EU firmware with it's native HWID 0896 and hope it doesn't kill it; or (ii) Leave it as it is and see if it can operate normally with the hybrid firmware in 'real world' operations.