I think that the same UART code for AT82S53 should work in your chip.
But AT89S8253 has 4 "fuses" (that is Atmel's name!): special flash bits that on programming allow some special options. One of them selects the x2 mode (microcontroller behaves like a 6 clocker). May be your programmer CLEAR that bit and enables de x2 mode. In that case, your baud rate will be doubled.

Is the chip transmiting? Can you see something in TXD with a scope?

Daniel