I would keep the Subaru letters, and remove the WRX badge, and whatever that other sticker is on the boot. Stickers/magnets are a bad idea on the surface of your car, not only can the paint fade a little bit around it so when you do remove it you'll see the outline where the sticker was. Also, they can hold moisture and mess up the paint and clearcoat.
A debadged car is a lot easier to wax, so besides a cleaner look, that's another advantage. Badges are pretty easy to remove, I've done it before with just some dental floss.