Once my wife was buying tyres and they put in the wrong number when the tyre store was checking on tyres and they were going to refuse to sell her the tyres she had requested until I pointed out that the higher speed rated tyres were for a different model, and she was requesting a tyre rated for her model, not the ultra performance version.
Frankly I would worry about a tyre store that would install a tyre that was below the recommended rating.Anything less than Z rated tires from Neiman Marcus would be so very gauche…
I’d second what others have said about handling, but if you don’t plan on going above the speed rating for the tires you get, there isn’t really a reason to get ‘Z’ rated tires. I’d visit tirerack.com and read reviews for the tires you’re thinking of getting. (by the comments here, it looks like you might have already done that)
Everyone is missing the point. Nobody should be going as fast as even a T rated tire allows. The issue is the handling dynamics of the car. The manufacturer has spent thousands of hours and millions of dollars to make make sure your car brakes, accelerates, and steers predictably in all circumstances all while getting the best possible fuel economy. You can save a few bucks and downgrade your tires but you will upset the entire balance of your car. You won’t notice it until one day you have to avoid a deer or stop quickly to avoid a running child. Then, as you crash, you will probably regret not getting the proper tires.
One piece of advice I give all car owners is that you should always buy the best tires and brakes possible. These are the parts that connect you to the road and stop you. You can buy cheap on virtually everything else but don’t skimp on tires and brakes. I have NEVER heard someone say they regret being able to maintain control of their car or stop more quickly. Just ask yourself, if your child was driving a car would you want cheap tires (or brakes) on it?