Have no problem with super-light 19s on 450+ bhp supercars 18s wouldn't fit over the huge brakes!
But on smaller, lighter cars 19s do compromise the ride and handling. Ask Stephen Marvin, technical director of Renaultsport. He says the Megane 250 handles better on 18s but the marketing department slaps 19s on all the review cars.
When Porsche sends a Cayman out for review it's usually on stock wheels: http://www.evo.co.uk/carreviews/evocarreviews/233378/porsche_cayman_s.html The Lotus Evora does have 19s on the back but they had the sense to put narrow 18s on the front.
I'm sure the 370 would handle as well as the 350 if they'd stuck with the same wheels, although they would look silly in those huge arches. Personally I'd rather have a car that's lighter on it's feet with a better ride and sharper steering, even if the ultimate grip is less.