M1,
Perfect corn is in the mouth of the eater. Perfect corn for me is accomplished by placing an ear of corn in boiling water for seven minutes and then promptly removing and serving.
As was said corn on the cob will continue to cook, how long it "holds" is beyond me. I would test it. I like the cooler corn method, again , test it on a few ears before you go for the big batch. Big batch cooking takes more into account, how much corn vs. how much hot water.
You can also grill corn, in the husk. You can parboil
http://www.wisegeek.com/what-is-parboiling.htm the corn and finish the cooking at the site. There's a lot you can do to forgo the headache of screwing the dish up.
I would definitely try the cooler method, with a whole lot more water and I would also do the parboil to cut down my cooking time. Up to you. I might also incorporate the corn into another dish such as "cowboy caviar" (google it).
Test the dish before the big day. Then figure out the ratio of water to corn so you know what to do for 30 ears of corn.
TS