You can also get 115200 baud with an 11.0592 MHz crystal using the baudrate generator on the P89LPC923

The question with high baudrates is NOT whether the UART can run at that rate, it is the code required to feed/drain SBUF that is the problem. If you try with a 12clocker at 11.0592 you will see that the code can not keep up with the UART at 115200.

The P89LPCxxx is a 2 clocker and can thus easily keep up.

Erik