Since you don't have the left side wall you won't get the left channels setup. The best mode in this case would be the 'My Surround' mode, which basically is the virtual surround sound produced by the soundbar.
In my opinion, unless you were planning on using the ysp in some other room as well, you'd be better off just getting a cheaper 3.1 channel soundbar like maybe the YAS 401 or an equivalent. It costs a lot less for probably the same sound stage that the YSP would have produced. That's my $.02
I have a corner setup, in a rectangular room with the couch 10 ft in front of the tv. I have no issues with the left channel but I don't have sufficient wall on the rhs behind me to give me a good right surround channel. In a corner setup I just have the '3 Beam + 2' mode. Which isn't all that bad.