Have you priced 19" tires versus 18s?
 

A considerable difference and one you'll pay every year or so. It isn't just the initial wheel investment....but the reoccurring costs.

What are your roads like? Smooth and you can run ultra-low profile tires. Not so good and you'll bend those new rims the first time you hit a bad pot hole. The lower the profile of the tire the less give there is when hitting an object before the object contacts something very very expensive.

I see lots of chrome wheels with problems. Repair of a "painted" one is cheaper too.

Don't change the 18"s. They look great. Perfect balance between looks, easy cleaning, performance, and comfort.
Definitely no chrome.
You could paint the side vents but it doesn't make much difference. The stock vents match the silver wheels, roll bars, and interior metal accents perfectly. No need for Cayman vents.

So basically leave it as it is. Perhaps, store it for the winter like some of us do, so it looks like a brand new car in the spring -- medicine against boredom and needless changes. :)

Z.

1. Go with aftermarket. That OEM 996.123.blah.blah.blah stamp isn't worth the extra bucks

2. Chrome is for muscle cars, stay with silver.

3. Paint the vents. Black brings attention to a feature that doesn't really need attention. It breaks up the beautiful flow of the body style. I do prefer the Cayman style too.