Hi Sean,
Bootloader doesn't restrict UART speed. Since Kinetis MCU UART module don’t have auto frequency detect function, the bootloader detects frequcny by software. It uses GPIO interrupt and timer to measure frequency. If the code can get the baudrate precisely, speed higher than 230400 should be fine. If it can't, you can appoint the baudrate in
bool scuart_poll_for_activity(const peripheral_descriptor_t *self)
Regards,
Jing