First of all, look here:
http://www.ft86club.com/forums/showthread.php?t=25177
You won't need spacers for 245's, but putting very wide tires on a car with as little power as the BRZ will just slow the car down.
Personally, I will be upgrading to 17x8 wheels and 225/40 PSS's. You want your tire to be stretched across the wheel and not be loose, but you also don't want it to be too tight or your ride quality will be harsh.