re: Are there any negatives to Z rated tires, for a sports sedan.

That's what it came with. And it was a non-Z car, so no inflator kit. This was in 03-04.

I haven't put run flats on anything since and just carry a kit, if it doesn't come with one from the factory.

I rarely drive over 80-85 on the street, but I get up to speed in a heartbeat.