There is no announcement yet on the next changes for the Z, let alone when they'll arrive, so it's probably not worth waiting at this point. If you want a Z, you should pull the trigger. Maximize your happiness now, or you'll end up making the same excuses every year for 10 years.
If you buy a used 09-12 in good condition and you keep it nice, it shouldn't drop down a ton very quickly at all. The Z is sweet looking, and more are retired to car heaven every year. Handles well, and has a nice power-to-handling feel.