DS89C450 from maxim
Yes, the maximum clock frequency is 33MHz, and one instruction cycle is 1 clock, but you are trying to compare uncomparable, the number of instruction cycles per a particular instruction is different for the RD2 (which is in this respect equivalent to the original 8501).
Although a precise ratio is impossible to give because it depends on the particular instruction mix in a given program, the instruction cycle/instruction penalty is around 30-50% for all the single-clocker '51 derivatives. It means that for the same oscillator clock, the DS89C4x0 would not be 6 times as fast as the 'RD2, but only about 4-4.5 times faster.
All this is valid with purely internal code execution with internal RAM. External memories access imposes extra waitstates on the DS89C450, i.e. even slower execution.
JW
PS. It's not from Maxim, but from Dallas Semiconductor - OK they merged so it makes little difference, just for the record... ;-)