Marshall wrote:
"You could discard the message after verifying that the message was not for the intended device but how would you know when to start looking for the next message?"

Erik wrote:
"With no relation to the MODBUS protocol, why do you not use the 9-bit mode unique to the '51."

Well, this Thread started with the 9-bit mode - and that would certainly do it: the setting of the 9th bit indicates the start of a new message, and this can be detected automatically by the hardware with no software overhead.
Variants with the "Enhanced" UART can also do the address matching in hardware.