I've never bought a new LS but I offer my thoughts....
It's reasonable to assume the "best" deals come on "end of model year" cars. So, if you can hold out till September, that would probably get your best deal.
This being said, I'd shoot for $5k off sticker, keep bugging the salesman for the next few months, and enjoy the process if you can wait.
By Sept., maybe you could get $7k off - I believe that's very acheivable...