Well, I can't speak for cars, but I know that my roommate had a truck that went through tires like crazy and siping them made them last at least twice as long. can't say for sure about what the performance on a car would be. they do run cooler which translates to longer tread wear.
Discount tired asked me if I wanted my Toyo's siped. I said hell no! Not for summer!
Just look at the Michelin Arctic Alpins I run during winter. That whole tire has millions of sipes (over exaggerating a little). It's nice and squishy for winter driving. It's a very soft pliable tire. Does great in the snow and tremendously well on ice. But it will get squirrely in the corners when they get hot.
The sipes will give you better summer traction for about a week. It basiclly creates a softer more plyable contact patch, but hard driving will chew them up in no time. I would recommend it for bad weather tires but not street tires unless you have money to burn or you get tires for free like me:nuetron: .