Rears definitely help.
I've been playing around with the arc and rears loads lately in different heights etc as we redecorated (reckon I spent a good 10 hours worth of testing, wife assumed I'd gone mad). I've found the sweet spot is to stick the rears exactly 12 inches (1 foot) above your head and to have the arc the reverse 12 inches below your head. If at all possible, have the arc completely in front of the TV, so nothing is obstructing it at all. I did have mine wall mounted, but we've changed furniture up so it's now on a unit. Obviously might be different for different room types and sizes, but was actually very surprised how much of a difference it has made, that it sounds like an entire new set again. Was very much for upfiring rear speakers as a new product, but not sure it's actually needed now as I really do get sound bouncing very nicely from the arc and due to the height of the rears you get that extra sense in overhead sound.