Will I regret this?

It means you need a smog pump with them.

The o2 provided from the smog pump helps burn off carbon and other materials that can build up inside the cat. So by eliminating the smog pump with a 3 way cat you can damage the cats.

Basically it's a package deal...3way cats and smog pump. If you don't want to run the smog pump, get the magna flow spun metaallic 2 way cats.