You can put on non directional tires do they exist on the rims no problem. I wouldn't rotate 315's with 275's. The reason why we have so wide tires out back is to keep the tail from coming around in slippery conditions/emergency situations. It also improve handling in general.
Going to a 20x10.5 or 20x10 all around setup is fine though. You will increase your oversteer though, but most likely without completely dialing out the factory understeer safe behavior.