Man, I think you should go for it! All the problems with the reflective sound in a small room ie; peaks and nulls vanish outside. I think a nearfield seating area would sound awsome with a 5.1 system outside. I would use a few Crown 5000w amps with some serious line array DIY speakers and a pair of killer dual 18" subs. I would like to do an outdoor theater also someday. Please, when you start post lots of pix.
Rick
Rick