tires

I believe most of the Grand Prix's have 225/60/16" tires. Is it OK to run 225/50/16's or will cause trouble?

when you run smaller tires all you probably need is to get a speedo adjustment (aftermarket electrical device that mounts in-line with one of the trans sensors on our cars) when you need to worry is with bigger wheels sometimes you have clearance/rubbing issues

Has anyone bought this size tires and if so, how big is the speedometer difference?

Nope. Too oddball. When your speedo reads 65 mph, you will actually be going 60.68 mph. One benefit is your acceleration may be a little bit better but fuel economy will probably suffer.