My company has been making DMX512 dimmers using 89C51 for 8 years and I have designed the hardware and written the firmware. I can't reveal much since there is a copyright problem, but I can say this :
1) DMX512 must transmit at 250KHz...no questions about slowing down or anything.
2) Use a 16Mhz xtal and you can get perfect 250Khz at the Tx output (pin 11) using the appropriate T0 configuration for Baud rate.
Sorry , but that's all I can reveal.