The bootloader loads before the firmware. So if you flash the tft files, you're just flashing over the firmware. The bootloader remains unchanged.
The only solution I would have is to try to get the phone in QHSUSB mode (or something similar) and then we'd have the ability to write over the the entire phone.
I personally don't know how exactly this whole thing works as this is my first Android Sony phone, but apperantly you can use programs like Omnius hack the bootloader. The problem is that the tool doesn't support the XS yet, but I'm sure once it becomes available we'll be told by one of the devs to specialize in this.