The strutbrace adds rigidity to the chasis, reducing flexing when you're driving hard, helps the suspension do its job. Ofcourse the strut brace doesnt do this by itself, it needs "friends" in other places on the car aswell.
But especially if you visit a trackday every now and then you'll want this kinda stuff on your car.
And no you dont need 400hp in your car to notice this.... :S