The ideal/expected placement for the speakers would be to the left and right of the viewing screen, in front of the viewer. If you place the speakers behind, you're not going to get nearly the effect. Honestly having the speakers behind you like that will be more, or as much, of an issue for the left and right channels behind you.
Personally, I don't have the speakers in the front for myself either, they are actually off to the left. However, TV/projector watching is very much a secondary purpose for the outdoor speakers, music being primary. The times I have used it is really to have a sports game on in the background or that sort of thing. If you plan on doing 'series' TV watching, I'd definitely look to put the speakers to the left and right of the viewing area.
Danny