How do spoilers affect performance on a front wheel drive vehicle?

I own a Scion xB (the boxy one) and was considering a spoiler, my vehicle is front wheel drive, and I have a lot of subwoofers in the back (weighing it down), I was considering the small one that Scion sells, but not the actual one, because it's like 450 dollars, and my friend can get one for my for like, 100, I was wondering if they would improve handling or performance etc.