Well I assume they designed the board for the highest rated chip they planned on using and just used a different chip and oscillator. Your options were 25i, 25, and 33 for the original rocket and only 40 for stage two.Has anybody considered or attempted to overclock a Rocket Booster? I have a 25Mhz Rocket Booster here and both 33Mhz and 40Mhz 68040 processors in my possession, and there's a nice big clock crystal can sitting right there on the card that looks ripe for swapping out with a socket or variable clock to play with.
As far as overclocking a 33 I would worry about power draw and any components that would not run correctly above 33mhz. Also if you have one of the add-on modules they might act goofy or not at all.
Not sure I would bother even if I could find a 40mhz 68040.