If I'm not mistaken the MKZ hybrid offers 19" wheel on it and gets 45/45/45. So expected reduced mileage and efficiency with your car. If there is anything mileage or efficiency-wise wrong with the car, should you take it in, the first thing they'll point out is the larger wheels. Be ready for that.
See if your dealer can do a $695 add on to your order. If not, look to buy fresh take offs from other Titanium owners.
Larger wheels have more mass and add more weight. The only way to overcome this is to have specially made wheels that are typically forged (aftermarket wheels) which adds considerably more cost. Also expect a harsher ride with the 19s (smaller sidewall tires). You will feel more bumps and such from the road. 19in tires are more expensive. Expect to pay $250+ per tire for new tires.
Otherwise do it. Your car will look pretty damn sweet and definitely not look like a hybrid! I was considering this as well since those 18" Standard Titanium wheels in my opinion look terrible. Not to mention will be difficult to clean. Either that or roll with the 18" SE luxury wheels (black or silver) or the SE sport wheels.
My advice: Keep your 18s and use those in the winter. Use the set of 19s for the summer.