I think the performance aspects have been covered very well here...so let me add a few other comments
You say you are looking for a 3 season car? What is your backup/winter car? A boxster is fine as a DD, going to work, picking up groceries, etc. But if you ever need to go and buy something large/medium sized - even a very small folding table, some chairs, or anything of moderate packaged size, you might not fit it in your car. Be aware of the flexibility you will give up! Its probably not something anybody thinks about when making this kind of decision. They say, "oh I really 'need' that sportscar, i'll make due. . . . ."
Also, if you are not mobile, very fat, or have a bad back, getting in/out or sitting in the boxster for a 10 hour roadtrip can get uncomfortable. And don't forget to consider other things - a box's stock sound system is terrible (make sure you get one that is decent, or don't like music
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