I know that the 87c51UBPN is rated at 33 MHz. Could it be pushed to 36 MHz for maximum processing speed without ill effect?
Usually parts spec fMax at min Vcc and MAX temperature.
That means if you control Vcc tighter you can gain more speed, or if you know MAX temperature will be lower, you can also gain more speed.
One thing to watch, is the fMax of the chip, can be higher than the Xtal Oscillator Fmax.
ie if you want to push to 36MHz, you may like to add an external oscillator.