From my point of view since you do not like the looks of the 17s so much anyway you should use them for your long trip or even ABUSE them if you know what I mean
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The 17s would have a more smooth feel on the highway than the 18s.
As for extended warranty I would not know it would be best to ask before retrieving the car!
Either way, I got a 986S and I consider it amazing, so I cant even imagine 987
Good luck!