Crelus,
I guess what I was thinking of was more simplistic than all that. I'm just talking about for example you've got a section of your map that has the large detention prefab. You try placing 3-4 goons with uzi's in there and test it. But, after testing it, you decide you don't like the goons with uzi's, you want goons with pistols instead. Easy enough to change, but most of the time will be in waiting for both those tests to load...and there really is no other way of deciding what you like better except play testing both types of goons.
In that scenero, I don't need to test the entire map.
But, I like your idea of the prefab functionality testing. It's a more modular way of looking at level design, so I'll have to change my thinking a little bit.
"Beware of people who try and belittle your ambitions. Small people do that. The really great people are the ones who make you feel that you too can become great" --Mark Twain