I think I should clarify my last post a bit better...for the record, I AM an advocate of the "long wall" set up for placing speakers! Usually, with less effort, you'll find you can get better sound overall with this set up. However, with your mentioning of setting the speakers all the way "in the corner", I was protesting a bit! speakers spread almost 30ft appart from each other in your set up, will indeed have solid immaging challenges.(especially in 2 channel). If you would move your speakers closer inwards toward the screen a bit, you would be much better off. If you have only one seat set in the middle of the room, you might play around with 1/4 possitions (definitely not my favorite chioce however) from the sidewalls, or slightly off of that(perhaps a bit better). If you want to sit in between the main two speakers for 2 channel stereo listening a lot, and the room is asymetrical, you have more options. So you'll have to play around a bit. If you have two seats you would be considering using, you might start with 1/6 possitions for your speakers along the long wall(from the sides). This is all assuming your room is a perfectly symetrical(and closed in) 30x15x9.
If you do opt for the long wall set up(which will require less immediate attention to critical room acoustics treatments and consideration, you will have a much better likelyhood of getting terrific sound.
I would however, if you can, strongly recommend you change the 15x30 dimmension! Bringing, say, the 30ft dimmension down by about 6% in length would yield you much much better sonic results than 30ft even!(assuming an exact 15ft for the short distance). The other attractive option would be to add 6% in dimmension to your 15ft dimmension! This will smooth out the bass nodes much better than a 15x30 dimmension overall.
Also, in reference to your SF Piano speaker sellection for HT applications, you might find the sound totally acceptable if you indeed do the "long wall set up", or at the very lest set up the system so that you're poximaly closer to the speakers...or at least closer in proportion to the speakers than you are to the side wall and ceiling reflections!
Anyway, hope this helps.