"I was told this option allowed you to "turn on" a sport option to stiffen the car up, etc. when you want to track the car or "go faster" on curves, etc. Am I wrong???"
Pushing the "Sport" button will change the throttle mappings and other engine variables, if you have PASM, it will also simultaneously enable "Sport" mode of PASM which tightens up the suspension. See here:
http://content2.eu.porsche.com/prod/boxster/boxsters.nsf/papenglish/chassissport_chrono
Wheels:
The problem isn't the larger wheels, it's the lower profile tires. Low profile tires are good, but not if they are too low. I don't have data either way, but it does make sense that the lower profile tires of the 19" wheels would be stiffer. In this case, just go with what you like.
Colors:
You're right, carbon probably wouldn't look too good with the terracotta interior... I've never seen one in real life, but looking at pictures, I can't imagine a brown interior going well with the dark grey exterior. I think it looks better with the colored exteriors (blue, red, yellow, green).
What about this combination:
Meteor Grey Metalic exterior, Black Soft Top
Two-Tone Black/Grey Leather Interior
Carbon Package
The two-tone interior looks awesome and really pulls together the outside colors. Plus, it should look good with the carbon. I just made one up in the Porsche configurator and it looks hot.
Headrest:
I also second the opinion that you should get the Porsche crest in the headrest. It's fairly cheap ($270) and really adds that finishing touch. It's more important than painted crests on the wheels imho.
Seats:
Before you buy anything, you really need to sit in each of the seat options. You will feel the seat more than anything else on the car, so it's very important you find a seat that fits you comfortably.