"Of course you can get out the assembler listing from your compiler, your datasheet and your calculator and do it the painful way... "

You can do this fairly easily in Excel: you just need to get the table of number of clocks required for eah opcode for your variant.
I've got one for the Triscend E5 on my homepage: http://www.8052.com/users/awneil/

Of course, you'll still need to manually work out the effect of conditional branches...